Not necessarily. Players put on IR after the start of the season can be designated to return after 4 games. I did not hear what Cajuste's injury was. In fact, I never heard mention of him being placed on IR until
@mgteich mentioned that Thornton took Hoyer's spot.. I went and checked the Patriots.com transaction page (which is not complete) and found Cajuste to the IR there.
EDIT: The NFL changed the rules this year in regards to the Injured Reserve List. Teams can now designate up to
EIGHT players to return. Previously it was
TWO. Players put onto the IR must sit out
4 games. It used to be that they had to miss
EIGHT games.