Monoprice via Rakuten Shopping offers the Monoprice CrystalPro 28" 4K 60Hz LED-Backlit LCD Monitor, model no. 12156, for $439.99 with free shipping. That's $134 less than what Monoprice charges directly and a very low price for a 4K display that runs at 60Hz. (Most of the less expensive models only support a 30Hz refresh rate at their maximum resolution.) It features a 3840x2160 native resolution (four times that of 1080p), 60Hz refresh rate, LED backlight, audio passthrough, DisplayPort, and HDMI.
Note that the 60Hz refresh rate is supported only on the DisplayPort inputs.