To be honest, it's kind of wary if we'll ever see Doom 4. Rumoured that it's in the works, I wouldn't be suprised if id works on that after Rage. But, I expect it to use Rage's engine. Why? Most games now are unpopular and low-selling because of linearity. Us gamers don't just wanna be the guy, from Red Dead Revolver (linear but classic PS2 prequel to RDR), we want an open world. We're demanding, and gamers would much rather enjoy an open world game than a linear one. That's not always true (COD, Uncharted, Final Fantasy XIII [Which some people DID hate for its linearity] and these games sell well also), but a lot of linear FPS and TPS get passed because of linearity. I would imagine Doom 4 would be open world, but then again, pople loved the Wolfenstein reboot (which I thought was a fairly good game), and it was half linear.
Let's just hope for a release soon.