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.