Does repeatedly saving a video degrade its quality?
MP4 isn't an ideal format for intermediate saves. If you know you'll be re-opening the file, save it as losslessly as practicable, and use MP4 only for the final output.

That said, depending on the encoder and settings you probably don't lose much if anything on subsequent saves. MP4 and similar codecs work by decimating the higher frequencies (details, resolution) in a predictable way. Once you've saved an MP4 at a particular quality setting you've already discarded the information. If you save again using the same settings, there's no information to lose in the areas that have already lost it.