like, the higher up you have to look, the smaller i'd make that monitor
my main row is 20" / 30" / 20" PLP with the dell 3017 in middle and 2007fpb on sides. i used my old lg 24" 1080p on top for glanceable info but i ended up taking it out, partially because it wasn't terribly justified, partially because i needed to use it elsewhere
if i had to re-add a 4th monitor to my desk setup i'd make it an ultrawide so that it wasn't as tall. and i'd even consider putting it *below* instead of above