Here is some nice 4K nature footage from Panasonic Sweden:
And a couple of 1080p clips from Eduardo Angel (from his post, "Panasonic Lumix DMC GH4 Sample Video - Behind the Scenes Fashion Week New York":
The resolution in the 4K clip is impressive (at least as impressive as can be seen on a 720p screen) - and the dynamic range in the 1080p clips seems decent (hard to tell without knowing how much light there was indoors).
Nothing here that would keep me from buying the camera - but anxious to see more footage.
In my view, Panasonic needs to get test cameras into the hands of more hybrid still/video camera bloggers!