This is pure rumour, but the Big K are reporting that a source inside Bethesda told them the company has made the decision to "indefinitely postpone" development of Doom 4. This move, made at a recent meeting in Dallas, apparently comes as a response to general disappointment with Rage, which was buggy on release and earned review scores of a less than 8/10 average (gasp!)
The story says that the issues surrounding Rage's launch have resulted in "a serious lack of confidence in the project management at id". This might be nonsense, of course, but it's believable nonsense. That said, I am not sure why the company would deny id Doom 4, which seems like a game that would sell no matter how it was managed. Also, Rage seems to be doing okay in the charts, and we all know that it's money, not review scores, that talk.