AMD FirePro, either a W5000 (DP 1.2) or a V4900 (DP 1.1), any of them fitted with an Arctic Cooling Accelero S1 Plus heatsink.
DP 1.1 can't drive 4k displays at 60Hz (only 30Hz). That said, technically the V4900 (using Turks chips) should be DP 1.2 capable, but I'm not sure the driver for this card actually allows selecting 4k resolutions without hacks - seems at least at some point it did not.
There's also a Club3d R7 250 EyeFinity 4 card out, which has 4 mini-DP ports, which should allow for 4 4k displays all at 60 Hz if I'm not mistaken. Certainly the 3d power isn't really all that great (still better than that V4900 above though) but if you're just interested in hooking up multiple 4k monitors for cheap this ought to do the trick. I fear though despite the relatively low power consumption it would need an aftermarket cooler since the heatsink/fan is tiny.