Can't say much without a picture but 4K definitely is not the problem.
Over and underexposed pictures or videos much depends on light and dark areas on your motive. Autoexposure function tends to make the whole picture exposed somewhere in the middle. If the calculation of the software is made on the central part of the picture which is usually the case, then only the center will be done correctly. Other parts of the picture if darker will be underexposed if lighter then that part will be overexposed. It usually happened if you are taking a video with the central dark area as in your case probably. Everything else will be overexposed so you must compensate that with one or two steps down. (exp.comp.)For the practical use: filming dark scenes I compensate that with two steps down (-0,7). If I take pictures of snow scenes then I compensate with one or two steps up (1.3 or 1.7).
If you have set the manual exp. 'ON', then you must choose the aperture and the time of exp. on your own. In the video, this is not practical as this needs to be corrected all the time when scenes change.