1080p ps4 pro games on a 4k tv?

I am looking at getting a 4k monitor for my pro, and i was wondering if i played the majority of my games in 4k but turn it down to 1080p boost mode for rainbow six siege, does 1080p look worse on a 4k monitor than a 1080p monitor?

I understand that it puts 4 4k pixels together per 1080 pixel, but is this really noticeable? Id rather have locked 60 fps on siege than 4k. If its relevant im looking at a 28 inch monitor that i would be about a metre from.

Thanks in advance