From a hardware point of view, the GH4 looks a lot like the GH3 - and they're similar enough in footprint that the forthcoming model can still use the same external battery grip as the GH3. Some of the fine detail has been changed, with the inclusion of a 1024x768 (2360k dot) OLED viewfinder panel and a shutter that's capable of operating at 1/8000th of a second and rated to last for around 200,000 exposures (twice as long as the GH3's).
Under the familiar skin, a lot else is going on. It's probably fair to say that the bulk of the camera's improvements will be most apparent to video shooters, but there are also some extra features added for the stills photographers that Panasonic says are equally important to them in the GH4. Based on our briefing and an afternoon spent with a pre-production GH4, we've summarized the changes we've seen from both a movie and a stills-shooting perspective.
http://www.dpreview.com/articles/9092895041/panasonic-lumix-dmc-gh4-a-quick-summary