Just thought I'd mention that Atlas apparently no longer sells pdfs through RPGNow.com - only through e23. The free 4e pdf seems to have been lost in translation... 
Yair
Any plans for anyone else to host such?
(So, if you've got a copy of the .pdf, back it up, before you regret it.)
From the frontpage:
One can hope "the rest" includes ArM4.
Over at the ENWorld forum, John Nephew said:
So, good chances for e23 to take it on. Although I'm not sure if it's geared for selling products at 0$, so maybe it will take some time.
E23 already has free samples of GURPS products, GURPS Lite for instance. So that shouldn't be an impediment to ArM4 continuing to be available. That said, I don't know what benefit Atlas gets by distributing ArM4 for free at this point.
Errg, it gets my undying gratitude ? 
Seriously, having the free game around allows one to set up games for people that are not familiar with ArM and don't want to invest in purchasing a new system's core book - but are willing to download a free pdf and read the parts you tell them to. That in turn should lead to purchases of accesories (including ArM4 pdfs), as well as transition to the new (and far improved!) ArM5. So the benefit is - more visibility, lower bar for introduction to the game, and of course ultimately - more sales.
At least that's my pitch.
I'm currently in an ArM4 PbP game - I'll ask who there owns ArM5 and/or hardcopy ArM4, it will be interesting to know.
Me, among others.
Shhhh! You're not helping! 
YR7's right- I'm guessing that, in keeping with the new wave of RPG marketing, it would get new players hooked on the basic concept/game world, and then, upon discovering that "expansion material" to that game is only available in 5th, they are enticed into buying the 5th edition.
Altho' "all" of us did so because we were playing 4th ed before 5th ed came out, the process for us was identical - a good thing getting better.
Would these possible benefits really be diminished if the game was available for $10, $5? or $2? I doubt it. I think most people who are seriously interested in the game would be willing to part with a few bucks to get ArM4.
I think Atlas might be better served following the GURPS model and putting out a lite version of ArM5 with some pared down rules, just the basics of spell casting, combat and character generation to get people hooked on ArM5 and cut the ties competely with ArM4.
Maybe I'm just prejudiced because the most visible user of the free ArM4, Abe, isn't adding anything to Atlas's bottom line, doesn't appear willing or able to ever buy an ArM product, and detracts from my enjoyment of this forum. (Although his confinement to two threads was pure genius and greatly reduces my annoyance with him.)
I actually went from ArM2 to ArM5. ArM3 was so bad that my group rejected any idea of using it and all of its demons. Then we went our separate ways before ArM4 came out. So, I've never played using ArM4, but reading it cover to cover on my laptop (more than once) did get me through at least two semesters worth of boring lectures.
I think in Abe case the key word is "able".
Somewhere I believe I read that the PDF killed sales of Ars 4 but likely helped Ars 5 sales as people were aware of the product and eager to see the new version.
I know somewhere, possibly at the old boards the people Atlas chimed in on this and provided an insightful explanation.
As a long time Ars player I believe the PDF made more people aware of the product and I know more Ars players "post" PDF than I did "Pre" PDF. Hopefully some of those people I've met are buyers of product.
As an active consumer, my observation is Ars is not only better in terms of quality, it's also healthier economically. If the PDF was a 'gateway" to Ars 5, then it did it's job.
3rd ed was the first I encountered, and so had nothing to compare it to except "other" games. Re those, it was so superior to anything else I, and my group, jumped all over it. That was just as 4th was being released, and those two were not so far apart (yes, yes, but not as far as expansion material).
I had massive quantities of 3rd ed stuff, and some 4th expansion, then moved and that was put in storage, and only had access to AM via the pdf. Since all my RP lately has been PbP, the pdf, both directly and indirectly, kept me playing, and in the loop.
I bought the hardcopy 5th ed only recently, and that was because I was still playing PbP, but more 5th ed games were surfacing. So, in a word, absolutely it contributed, it was practically the single contributing factor, and from PbP I can vouch that at least a dozen players I know learned from the pdf since 5th came out and enjoyed the hell outa it - whether they moved or will move to 5th I don't know, but they asked about it at the time.
YES. There is a huge difference between downloading a cheap product and downloading a free product - both in terms of psychology and in terms of process (no need to register your credit card etc.). I've seen this backed up by hard numbers (by Phil Reeds).
The point is to address people not (yet) seriously interested in the game.
I don't know. A pared down version of Ars Magica is such a different kind of game. A lot of the nature of Ars Magica is in playing with its flexible magic system and its seasonal activity method. If you do go down that road, don't produce a pared down system that doesn't allow for that.
Perhaps include a rudimentary advancement method based on post-gauntlet non-detailed seasonal advancement. In terms of magic system, perhaps a basic "ask the SG about the level, but generally magnitude 2 for almost natural manipulation, magnitude 6 for major magic, magnitude 15 for the most powreful of magics" or something along those lines (with some example spells and more details).
Doing a pared down version of Ars Magica right, i.e. in a way that will allow a player to play in an ArM5 game without too much trouble or to set one up without the ArM5 core book, will require a lot of work.
I don't think Abe is a phenomenon you should take into account in this decision, to put it mildly.
I am playing in the games that Yair and Cuchulainshound are involved in on the rpol.net site. I can assure you that a number of new players are getting involved in ars magica purely because they have heard good things about the system and want to play, but would not do so if it involved expense.
Once players know and like a system and intend to play long term then they will buy more products, but the new generations of PbP sites involve hundreds of game systems, and players who play online have so many options on what game systems to try out. When competitors offer free downloads to get new players interested, and you don't then they attract players. So far that has been working for Atlas.
I for one only got back into Ars magica after a seven year break from my university days because of fond memories and that the system was free. Playing it again, encouraged me to pick up the other pdfs that were available from RPGnow, so Atlas has recieved quite a number of purchases just from me due to the free book.
Without the free pdf being available, then it will be harder to attract new players in a completely online environment like the PbP boards. As other companies do offer it for their games and new players avoid expense for games they are not certain about.
As for a cut down version of the book, it is hard to see what could be cut down without skimping on the magic section which can be hard to understand anyway and uses up a significant portion of the book.
I play in a number of game systems, and Ars Magica is now one of the only systems I play in that doesn't release every book online as or before they release them hardcopy. I would be seriously interested in picking up the rest of the back catalogue of 4th edition books as well as all the new 5th edition books. Pdfs only is becoming more and more popular among gamers.
What books are missing? I think nearly all are, well were, on RPGNow, no? At any rate the Powers That Be have mentioned entering Atlas releases previously unseen in pdf into e23, so there is room for hope
5e is a different matter, with only a very partial selection of pdfs. I'm persoanlly satisfied with my hardcopies however, to be honest. I don't wish to spend money on purchasing the book twice, and having the hardcopy is neater.
Yair
In pdfs previously was all 1st, 2nd and most 3rd edition. But 4th edition was missing things like the tribunal books (only had novgorod and rome as far as i remember), the hedge magic, ordo nobilis and kabbalah books.
As someone who would be very interested in these missing books, I really would like them to be made available on e23.
e23 now does carry Atlas, including the 4e pdf: