Apple redid font smoothing with Mojave, and now it looks like garbage on non-retina displays. Most 4K displays seem to be bigger, rather than denser, so you can't do much to fix it.
Was this feature removed from OSX? Or did OP try it already and it didn't help much? Honest question here; I haven't touched a mac in over a decade.
Fixed the issue for me.
I've got an LG one at home, and heading into work where I have 2 27" monitors, but only 1440p always feels like a downgrade.