I just played the Quake 4 single player demo.
If this were an SAT test, you could sum up Quake 4 as
Quake 4 : Quake 2 :: Doom 3 : Doom 2
Basically, Quake 4 is Quake 2 prettified and given a new situation. You’ll even fight some of the same enemies from Quake 2 early on in 4.
This makes me sad, because the original Quake remains my favorite in the Quake series. I enjoyed the fact that the game jumped back and forth between military installations and gothic castles – it felt like you were jumping between two realities, ours and another that was attempting to take over. I am aware that this was because the Id guys never really finalized a design for the game, but I still liked it. I’d like to see a more direct sequel to the original Quake, perhaps with a feature where levels morph from realistic to fantastic and back while the player is running through them.