I think people are talking new gen XPS here right? Think all of them have supported 4k external @60hz as they have nothing older than broadwell.

I'm sitting here now with a gen1 xps 13 2015 pushing a 4k monitor at 4k 60hz.

My xps 15 in work also supports 4k @60hz although needs an annoying usb-c to displayport dongle.