Hi, on the S20 / S20+ / S21 / S21+ is 4k video with the 64MP camera only for zoom factor > 2x.
Its easy to calculate that the 12MP cameras have not enough resolution to produce 4k, they have 4032px in width, for 4k 3840px are needed, so it seems to be enough. But video without stabilisation is almost unusable and for the stabilisation 10%-15% of width are needed, the remaining pixels are not enough for a sharp picture. In reality, 2x zoom with the 64MP camera seems to have more resolution than the 4k video with the main camera.
3x zoom is useless again, the number of the pixel of the 64MP camera are not big enough to archive this.
So i think it should be possible to choose the 64 MP camera for 4k at 1x zoom, the stabilized and downsampled result of the full 64MP readout should produce sharp 4k videos.
We have really good hardware in the phones which are not cheap and there are always limitations in the software.
I have a second feature request for 64MP RAW in pro mode.