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Go to Apple Menu > About > System Report > Graphics/Displays. It'll give you two numbers "Resolution" and "Looks Like". The first is what's actually going over the wire to the monitor.
"Default for display" on my 4K is "Looks like 1920x1080", but it's rendered at 3840x2160 so you get nice hi-res text.
grams
"Default for display" just goes to 1920x1080
I think I've managed to make it work with the option key on scaled resolution. But, it does appear to slow everything down quite a lot.