I've only seriously tested the Dell P4317Q that I have in the office. Others have had good success with small 4k TVs. Can't say I've noticed anything about the latency, but I've never gamed or watched movies on it, so IDK
I use Samsung 4k TV's (55in and 43in) at work and home and the experience is absolutely fantastic. In game mode the latency is reported to be 11ms and there's no difference visible to me compared to 60hz computer monitors.
Do you have the specific model numbers? I'm buying a new monitor soon, and considering using a 4K TV. Would be great to check out the ones you're using!