Is this seriously a concern...?
It's already arguable that 4K resolution is even necessary on full size 50+ inch television sets with regards to difference in clarity at normal viewing distances and, hell, even the ability to tell them apart from 1080p panels. I have no doubt a 4K resolution looks better, but from normal 9 foot viewing distances in average family rooms with regards to adequate TV sizes, people can barely tell the difference between 720p and 1080p panels, let alone 4K.
The funnier part still is that we can't even fully utilize our 1080p sets considering most, if not all ISP's and cable providers, end up streaming much lower quality signals to your TV's while most, if not all, of the set top boxes can't even push what would be needed for the 1080p TV's we have now to utilize fully. The only way you're going to get such a clear signal is with physical BluRay media in home now, as no streaming or on demand video service can provide that level of quality, even as we enter 2013... and we're already thinking of 4K panels for TVs? Call me what you will, but that's putting the car before the horse if I've ever heard of it.
And 4K on a laptop, while not only being unnecessary from a perceivable viewing distance compared to the current retina resolution, would have the same issues. Any media you'd want to view couldn't be streamed to take advantage of such resolutions. Plus, any digital media isn't yet distributed, and probably never will be judging from how 1080p content has been handled, in any quality that will make that difference relevant.
TL;DR:
The current panel of the rMBP should actually be the least of your concerns for 4K resolutions. If you want to see the real problem, focus on the ISPs who currently offer no way to get content at that resolution efficiently (they can't even get full quality 1080p streamed to TVs in 2013, what makes you think they'll be able to push 4X the data in 10 years time), the media providers who are offering no good way to get media other than physically at that resolution, and the sheer fact that 4K vs. the current retina resolution of 2880x1880 on the rMBP could most likely never be differentiated at a 15" for factor, the concern for 4K resolutions and the current computer's monitor resolution should be the least of your worries.
Plus, by the time any of this 4K stuff becomes reality, the current rMBP's will be way past their usable lifetimes. I'm estimating mainstream consumer adoption of the tech in 5 to 8 years time... at a minimum...