Hate to break it to you but I believe (although I could be wrong) the 13-inch does not support 4K the same way the 15-inch MacBook Pro does. See here:
http://store.apple.com/us/question/answers/product/HD971LL/A/hi-i-have-a-macbook -pro-retina-13inch-late-2013-i-know-both-have-hdmi-ports-but-will-my-macbook-act ually-display-4k-onto-the-monitor/QX4HY7HTKUYTAYFUH
Are you using an HDMI cable or DisplayPort? If not using HDMI, you might want to try it. Or, per the link above, you may need to research what the necessary "hacks" are to get 4K working, perhaps at 50hz. I know SwitchResX is a common tool to make finely-tuned adjustments to the display output settings, including resolution, retina density, and refresh rates.