Retina vs. scaled for video playback on external 4k monitor

I have just got a LG Ultrafine 4K monitor for my MBP and wonder how retina on a 4k screen really works. The native resolution is HD but with retina doubling. I find a lot of confusion on the subject online…

Is it only for scaling text in a crisp way and maintaining normal window size, or does it also show a 4k video in quicktime for instance, at full resolution?

In short: Do I have to scale the monitor to 4k to show the quicktime video at full resolution? Or will the retina function also double the resolution for a video? Is the retina functionality controlled per app or all apps systemwide?