I read this info many times. Not sure if it right. If it is right, I would stay 1080p
You might want more than 1080 on 27"+ monitors. The question is if you want o jump all the way to 4K from 1080. The PPI on a 40" @4K would be the same as 20" @1080. There are other resolutions available to select for use on 27"+ monitors. Yes it is my opinion 4K is not needed until screen sizes exceed 40".
Actually my current monitor is 19" 1440x900. I asking; because I will upgrade soon. In my country (Egypt) almost all monitors are 1080p
A 4k monitor do not have a particular screen resolution.