Personally I find multiple "regular" displays works very well, because (a) there's much more choice of models (b) you have "retina"/High DPI options, and (c) IMO macOS works quite well with multiple displays, it works less well if you want to keep stuff organised across a single very big display.
It will depend a lot on your eyesight and your desk setup etc too. If you're sitting back a long way, the DPI on a 27" 4K may well be fine. If your eyesight is not great, thats possibly the case too.