I've read the GH4 is better in low light than the GH3. I have the follow questions about this:

Does the GH4 have better sensitivity at any given ISO?

Or Does the GH4 just have cleaner image at higher ISOs?

Do I have to shoot in 4K to get the better low light advantage?

I have two GH3s and I'm renting a GH4 as a third camera for a wedding. I'd prefer to shoot all three in regular HD but I'm hoping the GH4 will give me some low-light help.

Thanks!

Sensitivity is the same, the gh4 is only less noisy at the highest iso's + when you edit in a 1080p project and shoot 4K the downconversion makes the noise smaller as well which also helps in making it appear cleaner. I have no issues shooting at 6400 iso on the gh4.

Thanks Noa. So if I shoot the GH4 at 1080p, I won't see a low-light advantage?

From what I have seen, no, you only will have a bit less visible noise.