Quote Originally Posted by Iscah View Post
As far as I'm aware, there are no issues with using the same account on both PC and PS4, as long as you have purchased the game for both systems. It might get a bit fiddly if you bought expansions for one and not the other, though - I have a feeling I've read elsewhere that if you logged out in an expansion-only area, you wouldn't be able to log in on the other system that doesn't have that expansion installed.
This. You absolutely must buy ARR (base game / Starter Edition) for all platforms that you want to log in on at all. You don't have to buy the expansions for all platforms, but each platform keeps track of which expansions you have bought for that platform, and if you are missing expansions, you can only access areas and content that you have the expansions for on the platform you're playing on. If you log out in an area that you don't have access to on another platform, you can't log in on that other platform; so really, you have to buy the expansions on all platforms.