You go 4k if you want to look at a pretty game.
You go 144 hertz if you want to play more competitivly. Once you hit for example Global in CSGO something like a 144hertz becomes something of almost of a necessity. What a 144 hertz allows you to do is see the enemy on your screen quicker than what your normal monitor will (you've most likely got a 60 hertz monitor). This is becuase the refresh rate is higher which allows for this to occur. If you consider this option make sure that you actually have a good PC as you need conisistant high frames in order for the 144 hertz to take effect. If not, then you'll get stuttering.
On a note of the 144 hertz, i'd personally only get it if you're really good at a game for example like CS where you need quick reactions but other than that it's in my oponion pointless.
If you want some direction of what monitor to ideally get just message me and I can recommend a few.