The Panasonic GH4 has become one of my main shooting cameras. It has gotten even better, going into the superb, when adding the new Metabones 0.64 (Canon to m4/3rds) adapter.
The Metabones adapter takes most Canon lenses (see Metabones site for omitted lenses) and multiplies it by 0.64 on the focal length and the f stop. For example, a 100mm f/2.8 lens would become a 64mm f/1.8 lens. Of course, when putting any lens on a m4/3 body you still double the focal and f/stop specs. The advantage becomes a 100mm lens, rather than operating as a 200mm lens, operates as a 128mm lens.
This is all pertaining to a m4/3 camera and a full frame camera. If you add the crop sensor cameras (like the Canon 89D, Nikon D7200) into the mix, it becomes even more interesting. The 100mm lens put onto the Nikon or Canon crop sensor camera will operate at about 150mm. The 2.8 f stop will operate at about an f/4.5 f stop compared to the m4/3 results of an f/3.5 with the use of the Metabones on the m4/3 camera. The advantage shifts from using a cropped sensor Canon/Nikon/Sony crop sensor camera to a m4/3 crop sensor camera.