1Up has a nice Quake 4 preview.. and they warn that spoilers abound in other publications.. so beware!
Quake 4 PC Preview from 1UP.COM

Less than halfway into Quake 4, a game altering event happens. It’s one of those gaming moments that will be remembered and talked about for a long time. When it does you won’t be able to take your eyes off the screen, even though that little voice inside will be screaming “no, don’t watch!” We couldn’t resist, and as powerful as it was for us in a controlled demonstration, we can only imagine its impact in context while playing the game.

Yes, Quake 4 is shaping up to be that good. Good enough we felt we had to warn you about this. Not because we plan to spoil it (we don’t), but because it’s going to be out there in plenty of places and you may want to be careful what you read (and if you do want to know about it a sidebar in our sister magazine CGW’s June edition with Quake 4 on the cover spills the beans). But don’t do anything brash yet. We’ve got all the spoiler-free info you could want to whet your appetite. After that, well you’re on your own.