Bethesda has already dismissed this rumor via twitter: "Re: DOOM 4 rumor, games are done when they are done and no title under development at id has been postponed – indefinitely or otherwise."
This from the VP of PR and Marketing at Bethesda.
I have, and am really enjoying Rage, and so I would not consider it a failure. I have no idea what ID/Bethesda's expectations for it's sales were, though I'm sure EVERY company would like more than what they got.