Hi, The ultra96 board has a mini DisplayPort output that can drive a 4K monitor. But running full 4K video is going to be compute intensive. It depends on what you need to do. Generating 4K video from the PS (CPUs) is in general not going to run fast enough for a good experience.
The ZCU104 is made to excel at video, the Ultra96 board is more general purpose. The part on the Ultra96 is a ZU3EG the part on the ZCU104 is a ZU7EV. The 7EV has a built in high performance video codec enc/dec unit. The 7EV part also has more resources in the PL including UltraRam which the U96 does not have. The Ultra96 is a very capable board but you do get what you pay for vs the ZCU104 when it comes to video and then some.