[noggin]

maybe a 4k source downscaled to 1080p is better than 1080p 1:1
or not ?

where do you think the 1080p file comes from to begin with?

There are potential advantages in playing back a 4K file and downscaling rather than playing a pre-downscaled 1080p file that was originated 4K.

This is because the compression artefacts on the 4K file will be in the 4K domain, and so they will be less visible, and when the downscale to HD is performed, some of them will be rendered even less visible. The 1080p file will have 1080p 'sized' artefacts which could be more visible.

However down-scaling cleanly to avoid aliasing is a non-trivial exercise, so it has to be done well to avoid introducing artefacts of its own. (When broadcasters started producing in HD, they discovered that downscaling HD to SD was trickier than upscaling SD to HD.)