It'll be a while before we see Doom 4. In fact, according to id Software CEO Todd Hollenshead, they're still hiring staff for the project. But they do know one thing at least.

 "It's not a sequel to Doom 3, but it's not a reboot either," Hollenshead told GameSpot. "Doom 3 was sort of a reboot."

"It's a little bit different than those, and if I told you why, I would get my ass kicked when I got back. So I'll just have to leave it at that."

 Hollenshead said that fans shouldn't worry though, as everything he's seen of it suggests that it's "classic Doom." At the same time though, it's still "very much deep in development."

id Software has a lot on their plate these days, as they're also developing the multiplatform title Rage as well as Quake Live and Wolfenstein. The latter will be shown at E3, along with some "new stuff that we haven't announced yet."

 Even more projects? Busy indeed.

http://www.1up.com/do/newsStory?cId=3173721

very cool, i love doom. :)

So it's not a sequel, and it's not a reboot. What is it then? A prequel? Or is that considered a reboot?

Either way no matter what it turns out to be chances are it will be worth picking up. 

Good question though...

Maybe not an FPS? 

In other news:

Super Mario Galaxy "not a Mario game".

Sky "not blue".

Grass "not green".

Well as he said Doom 3 was a reboot maybe Doom 4 is a sequel to Doom 2?

no matter what it is, I'll be playing it... I really enjoyed Doom3

Unless Duke Nukem Forever comes out at the same time as Doom4, then I'll say goodbye to Doom4 (and all other games)

I don't care how they want to spin it chronologically, as long as it has:
Loads of demons.
Takes place on earth.
Is unashamedly violent.
Has awesome graphics (this one I can probably count on)

They should probably avoid calling it Doom 4 then. =P

Wolfenstein is gonna be awesome, and Im looking forward to Doom 4 as well because I like scary games.

Rage will be interesting because it's a new game engine from id.