Does Digital Movie IS affect resolution?

Hi,

I’m on my path to accept that IBIS is bad for video recording anyway and that Digital Movie IS is what I actually want – so that I can buy the R5C.

My question: I’ve always thought that digital IS decreases image quality (resolution), because the image is zoomed in. But is this really the case? When I record a 4K video, is it recording a smaller window (let’s say 80%) and then upscaling back to 4K, or is the camera recording 4K on a smaller part of the sensor, with a higher pixel density? The sensor has 45Megapixels, so that should be possible.
Thanks for your help!