Was at the Photo Show in Birmingham last couple of days and they had quite a few GH4s to play with, very impressive, and size wise it seems to be exactly the same as the GH3, so Zenmuse users will be happy.
The most impressive thing I seen today was the new Dual Pixel Auto Focus from Canon, you actually have to pay $500 to get this added to the likes of a Canon C100, but it is already in the DSLR 70D, I have not seen better focus tracking in any camera as good as this, theres no tracking back and forward, it just hits focus every time.