For web app development my setup is as follows:

4k monitor for two panes of code

4k monitor for a browser + debugger or maybe two browsers: one mobile one desktop.

1080p monitor for docs or maybe Netflix as I work depending on the kind of work.

When I'm doing robotics or server or more general programming work I basically just use my main monitor.

Reading code on a 1080p monitor is almost intolerable now that I'm used to 4k. I can size it way down and get two full panes of code side by side.