I just thought of some more games I forgot when I posted earlier:
Kameo: Elements of Power (2 player co-op for online as well)
Tales of Vesperia (apparently this has 4 player local co-op)
Green Day Rock Band (4 player co-op local and online, however local might be 6 because of the two extra mics that can be used)
Band Hero (I noticed it's under online co-op, but it has 4 player local co-op as well)
I'll post again if I think of anymore. Good luck!