Doom 4 comes after Doom 3. But is Doom 4 a sequel to Doom 3? According to id's Todd Hollenshead, no, it is not a sequel to Doom 3.
Speaking to GameSpot, the id boss said:
It's not a sequel to Doom 3, but it's not a reboot either. 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 arse kicked when I got back. So I'll just have to leave it at that.
Know what it should be? A sequel to the original. Do "Hell on Earth", except this time, actually make it look like Earth, and not Mars with a different sky texture. 2009 (well, 2010, more likely) needs rabbit heads on a spike!