I'm sure most of us here have already set some good examples of how he could return..
Summary in topic so far:
Screamking: Another video clip of him
Chrisi: Family kept him away and randy faked his death
Maloney: Flashbacks of Sidney or another character
Screamking has a good one there, another video clip is a good idea then again it has been done and he only done one video but i guess they could find another video he done maybe back in woodsboro or in Windsor college again.
I've heard the one what Chrisi said about his family keeping him "away" then again it's a good idea and a smart idea but they would have to have a good point about all this seeing as we witnessed his own death (which i think if we did'nt witness it they would probably use this idea)
Finally what i suggested is just the same idea basically as the video tape but then again there's a down side, they would either use old clips from S1 & 2 and maybe they'll just add his name on the credits (like they did on S3) or they might ask him to act out a flashback Sidney has that we never saw in S1 & 2 that they make up for S4.
~Maloney