Rumours are flying that Bethesda has temporarily postponed development of Doom 4 after the recently released RAGE didn't meet expectations. Apparently parent company Zenimax has "a serious lack of confidence in the project management at id," following the technical issues and luke-warm reviews surrounding RAGE.
We thought the game was above average in our review, but struggled to go particularly nuts over the game which cost a lot of money to develop. Sounds like Zenimax had much higher expectations for the series.
Bethesda's keeping schtum right now. We're sure there'll be more to this story as time goes by.