Echoing some of the comments here, part of this depends on whether you’re using MacOS or not and what else you’re using the monitors for. I use 4K monitors for both Mac and Windows and I stick close to the Apple recommended HiDPI approach which means your 4K display isn’t larger than 24” and your 5K display isn’t larger than 27”. YMMV but most people who have a bad 4K experience are trying to do 4K at 27” or greater. Due to DisplayPort 1.2 bandwidth limitations, the 5K 27” monitor market never got - great range of options. You either have the Thunderbolt-based monitors that are mostly for use in the Mac ecosystem (and only some Mac models) or a few expensive dual-cable DisplayPort 1.2 options or DisplayPort 1.4 options with an assortment of compatibility issues.