With OSX's pixel doubling, a 4K display is effectively 2k — 1920x1080@2x in a 15" would be nice, or even just matching the 1680x1050 in the 2011 HiRes 15"…
If you want a pixel perfect display all it can do is doubling. The scaled resolutions are more taxing on the GPU because it is essentially super-sampling - rendering to a larger virtual display then scaling it down to the actual pixel dimensions.There's no reason you couldn't do either of those "looks like" resolutions via scaling on a retina 15" display today; other than the fact that maybe(?) apple haven't put them into the OS. I can do "looks like" 1680x1050 on my 13" retina today. Not sure what the 15" does.
Retina doesn't just do pixel doubling, it's a little more complicated than that...
If you want a pixel perfect display all it can do is doubling. The scaled resolutions are more taxing on the GPU because it is essentially super-sampling - rendering to a larger virtual display then scaling it down to the actual pixel dimensions.
Personally I hope they put a 14" display in the 13" body (Less/no screen bezel) and give it a 3360x2100 display, giving it a "retina" usable resolution of 1680x1050. Also this will allow the smaller rMBP to have a scaled resolution of 1920x1080 (Although you can probably already do it via SwitchResX I imagine.)
I think for the vast majority 3840x2160 on a 15" won't be visibly different than 2880x1800 -- as others suggest, other manufacturers are just for bragging rights, since drain on battery to support more pixels doesn't seem worth it.
I actually run my rMBP 15" unscaled/native, but it's right at the edge of my visual acuity and if I'm tired its hard to read. Now if apple would release a 17" 3840x2160, that might be interesting....
Apple's mobile graphics can't handle 4K.
4K = reduced battery life.
Retina's good enough.
But I guess the answer to my initial question is no: there's no rumor pointing to a 4K panel, right?