Companions from former games will undoubtedly show up but which ones depend greatly on the setting. Thus far there has been no confirmation that I'm aware of that states the location in DA4 and it is only SPECULATION that DA4 will be set in Tevinter. Please do not spread said SPECULATION as gospel.
I for one speculate that DA4 will take place in Northern Thedas, covering the areas of both Tevinter and The Anderfels, much like Inquisition covered both Ferelden and Orlais. The Anderfels are Tevinter's direct neighbor to the west. If this is indeed the case then Dorian is the strongest prospect for returning characters. Whether he approves, disapproves or Romances the Inquisitor he states in each scenario that he will return to Tevinter. That, in my opinion, is a pretty strong hint.
Varric also states that, regardless if you save Hawke or the Warden Companion, that he will travel to The Anderfels after Corypheus in defeated. Another strong hint and in my opinion long overdue considering the many hints and rumors about what the situation is in The Anderfels and among the Grey Wardens. I can't imagine this being ignored for much longer.
Other characters likely to return in my speculative Anderfels/Tevinter:
Anders/Justice: They are tied to several on-going story lines, most significantly in this context is the rumor that Justice is capable of essentially immunizing Anders from the Calling. There are other reasons but this s probably the most prominent.
Fenris: Depending on his fate he is either chasing/killing Tevinters or already there but re-brainwashed. He will be featured in a up-coming book or comic(I can't remember the details) so we will find out if he's headed north or already there.
Bethany/Carver: If they lived, Varric says the Warden sibling is near the Andefels.
Velanna: She has her own hints. In all but maybe two potential endings, Velanna either ends up disappearing in the Deep Roads or her body mysteriously vanishes. I think she may return as part of the Grey Warden story line.