Yeah, what Lupus said - it's definitely not going to like Quake 3 (all multiplayer, no storyline). Looks like it'll be along the lines of Quake 2 as far as single-player goes, make of that what you will.
From what I have read and the vids I've seen (download the quakecon video) it will have strong single player which will play sort of like quake 2, but a fast multiplayer which will play like quake 3.
Quake 4 will be very similar to Quake 2. It will feature a fairly solid single player like Doom3, and a strong multiplayer. There are videos of this (not sure the URLs right now), but from what I've seen it all looks pretty good. Really more of the same, but that's what we want, right?