Computer and Video Games has posted that they have it "on sound authority" that Quake 4 is not only in the works, but has been in development for the past six months (thanks Stomped). They also say that this game is not actually being made by id, but rather is in development at a third party company, and they speculate (without any real evidence) that it may be an Xbox title. Keep in mind that there is no official word as to what id will be announcing at QuakeCon.