I liked Howard as Gwen a lot. In fact, I wish that she was cast as MJ in SM1 as it would've been better. But I don't think the Stacys need to return for SM4/5 if it really is the end of the run for the Raimi movies. If they are recasting and getting a new direction, I'm all for the Stacys to come back, but it was obvious in SM3 they were both stock characters that Ziskin forced into being names from the comic and expanded each of their screentime, but not their roles or character depth.
Raimi wanted to focus on Peter, MJ and Harry. Harry is dead, but I am sick of love triangles in the Spidey movies. If someone else comes and does it fine, but if we are really closing the door on the Raimi Spidey movies (with the possibility of Marvel doing it's own Spidey movies in 10-15 years?) then let it go on without the characters. Besides with GG dead, the most important aspect of Gwen cannot be addressed, especially since Peter is probably going to be married to MJ at the end of the last Raimi movie (be it SM4 or SM5).