Although both Rage and Doom 4 use the same id Tech 5 technology, John Carmack claims that Doom 4 will be so detailed that it will appear as if it ran on a "totally new game engine.“
Apparently, while Rage runs at 60 frames per second, Doom 4 is planned to run at 30 frames per second, which will enable id Software to throw "three times as much horsepower“ at it.
There's no mention of a release date, but since Tech-5 based Rage will come to PC, PS3, XboX 360 and Mac, that means that we’ll most likely see Doom 4 on at least two of these.
More details are expected at Quakecon event, toward the end of July.
You can read more here.