The RPi4 is actually fully capable of 4k @ 60Hz, as long as there is no 2nd monitor. If there's a second monitor, then two 4k displays at 30Hz each.
A 10" google search can yield https://www.raspberrypi.org/documentation/configuration/hdmi..., where it is all explained.
On a general note, the RPi4 is in my opinion the most exciting computing piece of news there was in 2019, plus the recent arrival of RPi4 8Gb gives the option for even more RAM. This is a sub-100€ quad-core RISC device that can surpass desktop class machines from a few years back in many ways.
The last 64-bit RISC device with a normal desktop I could sit in front of and work, was a Sun Ultra 10 back in 2000. I think the RPi4 comes pretty sweet close to being a normal desktop class machine.