No one's said that 4K displays are coming in 2014 rMBPs. In fact I sure hope they won't, because a 4K display in the 15" rMBP is equivalent to running in 1920x1200 scaled mode now, meaning very small user interfaces (too small for many users normally).
The rumor is simply that Apple may start using IGZO panels, nothing has been said about the resolutions involved. They could use IGZO panels with the exact same resolutions as the current panels, or they could upgrade to the doubled versions of the "high res" displays previously offered on MBPs (so 2*(1440x900) goes to the 13" and the 15" gets 2*(1650x1050)).
Anyway, just hypothetically we'll say there's a 4K rMBP with IGZO (maybe it's a 17", or maybe the next OS X version will address UI scaling to compensate for the tiny interface problem). 3840x2400 panel. We can get some idea of the power usage by looking at the 1920x1200 scaled resolution currently offered, as this already works like retina would on a 3840x2400 panel. So what you need is for someone to run a standardized battery life test at "normal" retina resolution and then again at 1920x1200 and see how much battery life you'd lose. Compare this against the expected IGZO battery life gain.
The test would need to be some kind of web browsing test most likely, as you'll want plenty of web rendering and scrolling to actually involve the OS's animation code. Anything without significant scrolling won't actually tell you anything about power consumption due to animation rendering.
