I've been using a 4k monitor for 4 years now (not that expensive - $300-400 for a 28 inch from Samsung) and just recently got a laptop with a 4k HDR screen. The big difference is not the increased resolution (I usually set it to 2560x1440 as it's less taxing on my computer and not noticeably different at that size). The big difference is that 4k has 10 bit color, as opposed to the 8 bit color that is standard for HDTV. That means about 60 times more shades of colors. Seriously, it's like night and day and I can always tell the difference between 8 bit and 10 bit.

Some screens also have 12 bit color now, which means about 4000 more shades than 8 bit. HDR is becoming standard, which basically means an increased brightness and contrast (I can already notice it on my laptop, but it's on the low end of the HDR standard so not much). And the new standard for color, Rec. 2020, calls for more than twice the range of colors than the last standard, Rec. 709 (HDR also has increased color range, but nowhere near Rec. 2020). This last one is what's giving companies the most trouble, but they're getting there, especially with the new QLED tech.