Hi,
I’m just getting back into Ars Magica after many years. The last time I played was in ‘94/’95 with 3rd edition (I’m dating myself). I just picked up the 5th edition core rulebook.
What 4th edition books are good books to get and compatible with 5th edition? I’m particularly interested in the Medieval Bestiary and the Wizard’s Grimoire.
The Wizard's Grimoire contains almost nothing that is applicable to 5th ed, except the purely rule-free parts (bookbinding, archetypes). Of all the other stuff, almost everything is either integrated - often simplified - into the core 5th, or contradicted or just not applicable. For 4th it was great, for 5th it seems almost useless. (IMHO)
I second all that.
But I still have benefit from my earlier edition books - but mostly for inspiration for stories, or the odd illustration, but any of the former bestiaries can be used for that (I have the 3rd and 4th - and equally like both).
If buying new books I wouldn't recommend anything but the 5th edition ones (unless you more or less already have the whole bunch).
I agree that the fourth ed wizards grimore is not going to provide any usable rules crunch. Furthermore Some of the 4th ed Wizard's Grimore material was not well playtested and is seriously flawed (specifically the spells and faerie magic ).
I'll also second the idea that the fifth edition books:
http://www.atlas-games.com/arm5/index.php
are a big step up from fourth edition books.
I think that hte most useful forth edition books are the ones that cover a specific subject in detail that we won't see again.
The following are books that I'd recommend for use (although you'll need to retool the mechanics for compatibility with fifth)
Ordo Noblis
Kabbalah
Heirs to Merlin
Land of fire and Ice
Blood and Sand
Medieval Tapestry
but I think that you will be on the whole happier with fifth edition books.
The fourth edition Beastiary is quite dry. It supplies lots of critter stats, it has almost no art (what art is there is not to my likeling). All of the critter stats would very likely need to be tweaked for fifth edition. You might be wiser cribbing from Mystery cults and the realm books. Make sure to download the book of mundane beasts from the fifth edtion product page.
I've tended to use the 4th edition version for crunch and the 3rd edition for fluff and inspiration (and the illustrations), but I guess having moved on to 5th the crunch is not as usefull any more. So if you want one -and you can get a hold of it- 3rd edition Bestiary might be inspirational. I know for one that the 'ant-lion' still gets me going every time (should think of a story at some point)!
I don't consider the Wizard's Grimoire to be a good book. It's full of unbalance mechanics and some bad ideas. It also has some good ideas and materials, but overall I won't recommend it (especially if you're short on money/time).
I likewise find the (4e) Medieval Bestiary rather bland. I won't really recommend it either.
I would strongly recommend purchasing the 5e books. They're great. Why are you centered on 4e books?
At any rate, the answer by book:
The Wizard's Grimoire, Revised: Mostly crunch that's not fitting with 5e, not compatible nor desirable for a 5e game methinks.
Houses of Hermes: Servicable for a 5e game, but the 5e House books are far superior and somewhat different (so if you'll use HoH you'll be playing in a non-canonical and worse game).
Mythic Europe: Don't own it.
Faeries, Revised: Although the stats will need conversion, and most of the rules probably too, it provides a fair amount of faeries and a vibe for what faerie is that's still very much usable with 5e (despite the great change in what faeries are in 5e). I didn't much like it, however. I thought it was overly filled with bad prose, not dense enough with ideas and material, and lacking and limited as a bestiary.
Hedge Magic: The book can provide ideas on hedge traditions, but I didn't particularly like most of the ideas there. I suspect conversion to 5e would be relatively easy, however (not sure, been a while since I've read it).
Ordo Nobilis: I just didn't like this book period, but it's perfectly suitable for a 5e game. If you want a book about noble's history, and for some reason don't prefer to buy an actual history book, you can get this.
The Mysteries: Although this version provides a little more fluff and flavor of the magical traditions, and was great, I would recommend getting The Mysteries Revised Edition for 5e. It's far more suited for a 5e game.
The Medieval Bestiary: I found it bland and dry. The statistics would also need a lot of conversion, which doesn't make much sense. I would recommend getting the 5e Mystery Cults and downloading the Book of Mundane Beasts instead - that gets you much of the content. You can gleam lots of ideas on magical beasts and animal-related symbolism and magic from this book, however, so perhaps you should consider it.
Living Lore: Don't have it, but I suspect it's pretty applicable to 5e (although mechanical covnersions would be needed).
Faerie Stories: Likewise.
Kabbalah: If you want a primer to mythic Judaism, this is an excellent book. It doesn't seem likely to be relevant to a saga, however, and the mechanics would probably need an overhaul.
Ultima Thule:Don't own it.
Parma Fabula: Irrelevant.
Triamore: Don't own it.
A Medieval Tapestry: Don't own it.
The Mythic Seas: Don't own it.
Mistridge: Don't own it.
Mythic Places: Don't own it.
More Mythic Places: Some nice ideas. Need lots of conversion, but some nice ideas so well worth it for any edition IMHO.
The Medieval Handbook: Don't own it.
Pax Dei: Don't own it. I suspect the 5e The Divine is far superior.
The Maleficium: Don't own it. I suspect the 5e The Infernal is far superior.
Sanctuary of Ice: A reasonably good Tribunal book. Some details don't fit the 5e cannonical view of matters, but that can be skimmed. If the place and atmosphere appeal to you, I think that's a good choice. (I don't think we'll see any tribunal redone.)
Land of Fire & Ice: It's an interesting high-level ArM saga, off the beaten path. Converting it to 5e will require a lot of work, however, and some thinking too due to the change in what Realms mean. I don't know.
Blood and Sand: Don't own it.
Heirs to Merlin:Don't own it.
The Dragon and the Bear: A 4e tribunal I very much love, I think this would serve well for 5e (although you will need converting the pagan priests and hedge wizards). A lot of work in conversion, but a lot of good ideas and an interesting place IMO. (They botched the whole Mongol issue, though...)
Tribunals of Hermes: Iberia : Don't own it.
Tribunals of Hermes: Rome: I didn't find it particularly appealing, and also a rather small and crowded tribunal. But I think it's largely still usable.
Lion of the North: Don't own it.
Cause & Cure: Don't own it.
The Black Monks of Glastonbury: Don't own it.
The Return of the Stormrider: Don't own it.
Festival of the Damned: An interesting adventure. Will need pretty heavy conversion to serve for 5e, but I think it can be fudged into place and be fun.
The Fallen Angel: Didn't like it. I think it's pretty applicable to 5e though, as much as it is to 4e.
The Bishop's Staff : I think it's as workable for 5e as it is for 4e, although I don't remember how much conversion would be needed.
Are the 4th and 5th Edition Mysteries completely different or just a rewrite/update of the same material? I mean, is it worth buying the revised edition if you already own the 4th ed. one?
I would advise you against both of those, I'm afraid.
The good ideas from Wizard's Grimoire have been re-used in spirit (updated significantly in mechanics) in Covenants and the Houses of Hermes books. Covenants has the advanced book rules and the House books contain most of the legal, political, and social information on the Order.
As to the Medieval Bestiary, I just don't think it was very good. 
And as others have pointed out, the mechanics are dated. You are better of with Houses of Hermes: Mystery Cults, where the House Bjornaer section gives rules for designing your own beasts.
You asked about Mysteries and The Mysteries: Revised Edition. I am not fond of Mysteries myself but the Revised Edition is significantly different; a bit less flavor but the mechanics are much better. My opinion only, of course.
Completely different. I gather from the comments of the authors that the fifth edition book relied on significantly earlier sources (closer to the default time period). The mechanics are completely different. the cults are different. The virtues are different.
The fourth edition book was a bit more occult than the fifth edition, it put a high priority on the mystical flavor of the text (which made it a tough read in some places). The fifth edition book is significantly better play tested and the text, while not as rich in feel, clearly sets out some mechanics that work marvelously well for making interesting stories.
A fellow named Wanderer posts here often with posts showing that he uses the rules much differently than I do and in a manner that I would personally find tasteless and annoying but he seems to enjoy. That the rules accommodate us both is testament to their adaptability which may or may not be a benefit.
Buy 5e--they are good to be honest.
I liked it when it came out. It gave me leads towards what I actually needed instead of letting me flounder about (as is all together too easy). It's also geared towards gamers, which is nice.
Similarly...
...was a personal favorite for a long time.
I think The Mysteries: Revised Edition has awesome mechanics. I don't love all the cults, but the scripts mechanics (mechanics for designing non-hermetic cult rites, rituals, and ceremonies) are awesome.
Biggest changes in 5th ed are a "grand unified theory" approach to the lab and spells (one formula, no waiting), and the ability to CharGen a reasonable, in fact quite satisfying mage of any Hermetic Age (100 years after Gauntlet, no sweat - well, a bit of paperwork, but quite doable BTR.) Weapons combat is both downplayed and streamlined (yay!), and magic is both better explained in depth and more self-consistent. Various details throughout 4th that were problematic have been ironed out, and CharGen is a more thoughtfully presented process.
Both editions follow the same general approach, so there won't be a huge paradigm shift there, but the details of the mechanics seem to work more smoothly. At a glance, you'd have to know where to look to tell a 5th ed charsheet from a 4th.
While I am a big fan of 4th ed, and would still play it today, I've got to admit that 5th seems to be better thought out over all.
(I do prefer many elements of the old 3rd ed Covenants over 4th or 5th, just seems to work better for my gaming style, tho' ideas from 3rd, 4th and 5th are all grist for the mill.)
IMHO 5th ed. Mysteries is really a must have book - if moving beyond the Core Book. It's the universal toolbox (Though I know some people wanted more finished cults than tools alone when it came out) for a lot of other books - and I think it's quite a part from the earlier version.
Kind Regards
Jeppe - who just (finally) digitally picked up his copy of Mysteries for MetaCreator 
All of the highly useful bits of The Grimoire have been recycled. The book binding stuff was actually broken, although no-one seems to have been all that bothered by it (although the hole has been patched in the new version of the rules for doing the same thing.)
The Bestiary is...well...it's OK. It's not as good as the first one (2nd edition) for colour text and although its rules made a lot more sense, they've had their time. I wouldn't buy it now, though.
I think that there are several 4th ed. books that are still very useful, despite the changed mechanics. If your wallet allows, you could pick up the ones you find most useful, particularly any of the ones that might describe the tribunal you wish to play in.
My personal 4th ed. favorites are:
Land of Fire and Ice - because I love Icelandic sagas and the book does a great job of capturing the feel of them. There are two sagas included as well, which serve as a great example of using the unique features of a place (Iceland) in a saga.
Heirs to Merlin - I slagged this when it came out because it didn't have stats, but now find it perfect in that it doesn't have stats but still describes Mythic England very well.
Lion of the North - the book is very well written, in my opinion, and describes some very interesting covenants and their relationships, all stacked on top of an ancient form of magic.
Triamore - never posted as a favorite, but I like it because of the detail applied to a single covenant and the surrounding area. The characters would need to be revamped, but much of it could stand as is. it's a great starter covenant.
Faerie Stories - several simple, fun stories, set in a specific location, that are easy to run at conventions and as filler for the troupe.
Matt Ryan
I readily concur. My saga is running on the 4th year in a covenant that's a remake of Triamore placed in Stonehenge with plot hooks tied in with Scotland too. Those books did a big part in me ending up with a saga there.
Placing triamore in a border region (any border region) between contending nobles makes perfect sense. Good idea there Furion.
Cheers,
Xavi
We're actually situated in Shropshire on the border to the wild uncultivated lands of Northern Wales - but some story arcs are connected to the time of Pralix and to the areas to the North of Stonehenge.
Our covenant, Ianitoris Oporotheca, is as a manor (too) deeply involved in Mundane affairs but not quite as centrally placed as Triamore originally.
Well, calling triamore a "manor" is quite downgrading it, don't you think? it is an awfully large castle! Ruinous, but a castle none the less. And one thatr magic can complete into fullness (for moon duration at least) rather fast in case of being attacked. 
Cheers,
Xavi
I think I have some photos of the castle at Clun (or is it Bishop's Castle?) as it is today if you are at all interested. 
cj x
Xavi - you are right of course, but I think you are misinterpreting my use of the word manor. I was using the word as in estate, territory or demesne and not as in the sense of mansion - a dictionary will tell that it can be used as either. 
In fact the unfinished nature of the place is one of our active story arcs at the moment. On one hand the grant from the king to our covenant specifies that the castle cannot be completed or extended but on the other hand the magi would like to do just that. Not least due to a siege during the civil war as a neighbouring lord tried to use the upheavals for his own designs (during which they certainly used Creo-magic to strenghten the defenses). At the same time one of the newly gauntleted magi (a PC) was denied joining the covenant with only one vote apart - his own former master (No, he is actualy not Tytali - but he is Ex Misc with Diedne roots having a particular take on how to force his apprentice forward) - but he was promised he could join the day the castle is completed... As much a political as a financial and architectural challenge!
CJ - certainly! I'll message you my mail.