I went for a 144Hz and dont regret it at all. If you play shooters, 144Hz is a biiiig difference. Went from silver to GM in Overwatch after I got a new monitor (Though probably not only because of the monitor.. )
There isn't really any need for 4k unless you are extremely uptight about graphic settings. In my experience, most people who play online games are more concerned of smoothness of gameplay than quality of the graphics.