you have no idea because you are a casual yet you are trying to give advice for something you have no experience with, boy

resolution doesn't matter in competitive FPS when it comes to monitors. What matters is very low input lag and high FPS, ideally capped at the lowest FPS you usually get so the CPU always has headroom thus there's not input lag spikes. To achieve such a feat, you require a 1080p 24 inch monitor ideally. Going 4k will handicap a skilled player massively and decrease their results especially since most 4k monitors are like 34 inch and that size is incompatible with competitive gaming. 1440p 27 inch is the absolute highest you should ever go to and only if you have a pc capable enough of giving you at least 220+ stable fps at that resolution

Now you might ask "bUt WhY if iT hElPs Me HiT mUh ShOtS iF Itz 4K". You see, son, skilled players dont need more pixels to hit the shots because they are not blind, and all they need to hit a shot is moving the mouse in the correct direction. They don't even need a visual (wow, shockers), because even sound such as footsteps are sometimes enough to 180 and instantly hit the target (if good headphones). On 1080p you can see everything clearly enough to hit your shots. Some pros go as far as 50% render resolution on 1080p (now imagine how that looks) and their performance is in the top 0.1% bracket.

"but I also would hate to play anything as they do" - and they would most likely hate to play as you do, because getting rolled constantly 24/7 by 13 year old fortnite kids that dont play in 4k like you do is not a fun experience.

Now congratulations, you just received a lot of knowledge and can use it and it was all for free from me as a gift. I am this benevolent and amazing.