While your numbers are technically correct, that’s not how it works.
4K is 2 x 1920 x 2 x 1080 basically 2 x 2 1080p.
50% of each side of 4K cuts it in half, which gives you 1920 x 1080. That’s how resolution scaling works.
Just use the slider in the menus and adjust the scaling. You’ll see it matches up exactly with what I’m saying.