Welcome to an insight into the 4K Video mode. When you downscale from 4K to full HD, you're essentially oversampling the image to have 4x the data for every pixel. Therefore, when you have 4K footage and downscale it to 1080p (Full HD), the image is going to look better than it would at native 1080p. Do you also own an X-T200? Write your experience in the comments :)
Camera: FUJIFILM X-T200
Lens: XC15-45mmF3.5-5.6
Portable Tripod
Music by High Life - The Mini Vandals featuring Mamadou Koita and Lasso