4k movie on 4k screen scaled to 1080

Question

I have my first ever 4k screen - a new 55" TV - & I'm struggling with this paradigm.

I have to set it to show the desktop at an apparent 1080p otherwise I need to be within about 15" to be able to actually read it.

So, what happens if I want to watch a 4k movie on it?
This is a bit hypothetical at the moment, I don't yet have any 4k content to test this, & I've been holding back because I'm thinking it's just going to get down-sampled to display on my "1080p" output, thereby gaining nothing.

Is this actually going to be the case? Am I going to have to re-scale the screen in order to watch at 4k, or will it auto-magically display the desktop at 1080 but a fullscreen movie at 4k?

I'm currently using VLC as playback app, set to not use macOS native fullscreen, so it expands in the current Space rather than making a new one for itself.

Mac is an old Mac Pro 3,1 stuck on El Capitan, with an aftermarket 4k-capable GPU, connected over HDMI & working entirely as expected for 1080p & lower content.

To clarify, all scaling is being done on the Mac, as though it was a retina display [every other display I've ever used is non-retina, so this is all very new to me.]

_{Pic taken over ARD, too lazy to walk to it right now ;)}

Answer 1

OK - asked, tested and answered…

With the screen set to 1080p, that's all it will play at, whatever the source.

To get 4k the screen must first be set to 4k.