What lens and settings did you use (aperture, iso, shutter speed)? Color grading software? What did color space did you output to? Do you have a calibrated monitor? if Rec709 there would be information loss (likely darker).
Too dark is relative. I suspect performance should be better than GH4 just on sensor size and raw (unless GH4 external recorder).
Anyway thanks for sharing - night time out doors with video projection seems like a difficult subject regardless.
I have used f1.5 Rokinon for night shoots and can get a lot of information out of the blacks. But for my f4 lens it would unpleasantly noisy.
I am studying Japanese and should have known the Disney characters would be speaking japanese but still pleasant surprise

I suggest trying to photograph a person next to a strong candle light as a test. Too dark is not usually the issue but rather too much noise in the darks.
ありがとうございました
Mike