You also have to consider how the camera is generating the 4k video.
Often, even on full-frame cameras, 4K is obtained by using only a cropped 4k area, instead of the entire sensor.
Other cameras with faster processors can perform on-the-fly downsizing from max resolution down to 4K.
If that's the case, a full-resolution video downsized to 4K on a smaller sensor, might be better than a "cropped 4k" from a full frame camera.