The reason 4k monitors only support displayport is because current HDMI can only do 4k at 30hz. DP is a better cable with more bandwidth. Expect DP to overtake HDMI in the future.
I have a Vizio P55-c1 on my desk and with a GTX1080 and a BlueJeans HDMI cable, it works fine at 1080p@120hz or 4K@60hz. It's HDMI 2.0, so another case where just because it plugs in doesn't mean it'll work: https://en.m.wikipedia.org/wiki/HDMI#Version_2.0
