It's a known issue that latest version of Mac OS has problem to connect to external 4k monitors. I did not experience kernel panics, but lagging on display, high memory and cpu usage on the windowserver daemon.
I thought it was just a combination of low-end MBP (cheapest 13" available) and crappy 4K monitor (Sceptre U274K). Are you saying that there is an actual problem with macOS High Sierra that is causing my lag? I've already changed DP cables and that didn't help at all. :(