Hi Charlie,

My take is to use mp4 from Cinema 4D for previews only, if at all.

Get the best material possible out of the render engine. Then, use this in an external app to fine-tune it to your needs.

To re-render just to test another mp4 setting seems counterproductive.
A resulting darker image can only be checked with a project file.

I can take a look, but perhaps tech support is the direct way when you think it is not based on your settings.
https://www.maxon.net/en/support-center

All the best