John Carmack confirmed in a recent QuakeCon interview that Doom 4 is in the future plans of id Software. Says he:
There will be a Doom 4, we don’t have it scheduled or a team assigned to it, but there will be a Doom 4. There’s going to be a Quake Arena sequel. There’s a Wolfenstein thing in production. We’re following along with all those. This game doesn’t have to be Doom. It’s going to be something different.
You can read the whole interview here although there is nothing else interesting of course.