If, by 4K, you mean UHD at 3840x2160, my unmodified PC variant of the GTX 670 SC works just fine at 60 Hz. SST via DP to the new Dell 24 inch UHD monitor (P2415Q) on my 4.1 (flashed to 5.1). Booted right up in Win 7 at native rez. In OS X, apparently Apple decides what is best based upon screen resolution and size. Therefore with a relatively small UHD monitor, it defaults to 1080p Retina, and native rez cannot be seen as a scaled option either, unless you press the scaling button AND the option key. Then, just click 3840x2160, and you've got it running natively at 60 Hz. The 27 inch variant of the same monitor does not seem to have this issue. UHD appears as an option without using the option key.
OTH, if by 4K, you mean DCI 4K, that's an acquisition format and has, at present, a single common use: one to one pixel viewing of digital footage acquired in 4K. In all but what might be the rarest of cases, that eventually gets horizontally or vertically cropped to the final aspect ratio. Don't let consumer monitor and TV companies trip you up with this "true 4K" hoopla. For all practical purposes, both resolutions are "4K" but are not equally supported. There are two DCI 4K monitors and maybe one DCI 4K TV. Apple did recently do something with LG to get the LG 4K monitor working in OS X, but officially, only at 50 Hz. I did see the OP is in LAX, so maybe he's one who actually uses DCI 4K.
There are plenty of UHD "4K" 3840x2160 monitors out there, with more on the way. Name brand UHD monitors with IPS panels, SST at 60 Hz., can now be had for down around 500 bucks. The Dell I have is own sale for 529.99 USD last time I looked this week.
