Doctor Gordon won't be coming back, ever. unless they cast someone else to play him.
Saw 4 won't be picking up where the 3rd one left off. from the forums on saw3.com, the official lionsgate press release states that Swat captain Rigg (from part 2 and 3) will be the main character of the movie. he is given 90 minutes to go from location to location getting involved in life or death games to save an "old friend" (rumors on the saw forums is that the "old friend" is detective matthews)
time is played with alot in this movie, as mentioned, but somehow John "Jigsaw" 's ex-wife Jill is involved with the movie somehow, and very heavily from what i'm reading. John/Jigsaw IS dead, thats confirmed and he's not only limited to flashbacks. rumor is that his new thing is "videos" a video describing how the traps work, and what Rigg needs to do to save the victim (all interconnected, somehow) what they are saying is that he made the videos and set all this up because now that he is dead, he isn't worried at all about people seeing who he really is. what are they gonna do, press charges on a corpse? no need for the mystery anymore, he's dead and carrying out his last games with video messages now. (perhaps set in place by his ex-wife?) it is not mentioned how, exactly, she is involved. but it is mentioned that she plays a very large role in the movie. we'll find out more in a few weeks when the first official trailer is released (supposedly in the first week of august)