Oh wow you are right, I completely forgot each independent monitor can be adjusted. So my first two 5k monitors can be set to "Looks like 2560x1440", while the third 4k monitor can be set to "Looks like 1920x1080", and then this will give me perfect 1:1 scaling of the 4k clean feed video. Thanks for this hint!If you buy a 4K monitor you don't have to run it with a 5120x2880 framebuffer!
If you choose native 3840x2160 resolution or "looks like 1920x1080" you'll have a 3840x2160 framebuffer and of course get a pixel-perfect 1:1 rendering of a 3840x2160 video in full-screen.