Display port gives 60HZ, HDMI only 30HZ?

This is a limit of HDMI bandwidth, it can only do 4k at 30hz. The new HDMI 2.0 spec products are just starting to come out now and they can do 4k at 60hz. Your product is a first gen 4k monitor so you pretty much have to go with DisplayPort as 30hz is almost unusable.