I hate making a topic just for what is probably a simple "yes" or "no" question, but I'm not sure how else to get an answer:
Are the tribunal books from 4th Edition still viable from a Lore perspective? It seems a bit unlikely that we are going to get a Stonehenge or Greater Alps (Alpine Tribunal in 4E?) book, and I just noticed there is one for Iceland, which I was interested in.
Heck, I don't even know if 4E starts in 1220 or if I'll need to extrapolate. I know that some game lines like to move each new edition forward by some number of years, did that happen here?
It depends very much on the tribunal. Blood and Sand, for example, presented one set of lore for the Tribunal of the Levant, but much of that has been changed by the updates made in Realms of Power: Divine, The Cradle and the Crescent, and Lands of the Nile (it may be the most rewritten tribunal at this point!). This included completely removing a Hermetic Embassy in Baghdad, among other things.
On the other hand, other tribunals have remained relatively untouched since Third Edition, except in as far as some aspects have been changed in a backhanded fashion due to things found in, for example, House books.
Of course, it's your saga, so you can of course use or discard as much or as little of the older books' lore as you wish. For Iceland, the book you want is Land of Fire and Ice. Ultima Thule would also be useful, given the close connections between Iceland and Scandinavia.
It is not a simple yes/no question. The answer is somewhere between maybe and «that depends».
IMO, you can generally use 4ed books, but you have to be prepared for inconsistencies. Even if you stick with 5ed canon, you have to be prepared for inconsistencies. Contested Isle lists all the covenants of Hibernia, but there is obviously one missing, which is given in that book of fully statted covenants, and it adds a representative of the line of Hibernian sahir-like magi. Inconsistency and ambiguity is part of the game, so one just has to deal with it. Generally the game also gives troupes a lot of leeway in how to interpret houses and traditions, and the level of magic (and vis) in the world. Some of the inconsistencies can be smoothed over by exploiting that freedom.
Personally, I am quite happy with using Heirs to Merlin, but there are two things which have to be rethought.
The way House Tremere and the Tremere-dominated Blackthorn covenant is depicted. Older edition tends to make Tremere more antagonistic than 5ed does, and in Stonehenge, Blackthorn definitely is an antagonist. Whether this is a problem depends on how you interpret Tremere in your saga.
Redcaps. Stonehenge has a lot fewer redcaps than 5ed assumes, and they deliver messages more often. Running the numbers, none of them have two genuinely free seasons per year as they are supposed to. They also retire a lot earlier than one would think with 5ed longevity rules. One may also question their all having Leap of Homecoming devices with 5ed warping rules, but I guess they could have them tailor made and thus not warping.