According to a rumour picked up by Kotaku, publisher Bethesda has put the breaks on development of Doom 4 at id Software. According to the source this was mentioned at a id meeting in Dallas recently.
The same source claims that the mixed reviews and limited commercial success of Rage are to blame, but at this point there is no official word.
Doom 4 was first announced back in 2008, and believed to have been in slow development during id Software's completion of Rage.