The GH4 is, I think, perceived mainly as a video camera. As such it probably tends to get overlooked when it comes to stills.
I own the GX1, GX7 and GH4. I bought the GH4 primarily for 4K video. I must say that the 4K video from this camera is nothing short of stunning. I have a 28" 4K monitor on my computer, so I can view the GH4 video in all it's glory, and it is very impressive.
Since buying the GH4 I have not picked up any of my other MFT cameras. Stills image quality is easily as good as the GX7. I should state I only shoot raw, so I have no idea what incamera jpgs look like. The camera gives up nothing in stills image quality to any other MFT camera, in my opinion.
Any differences in image quality would require extensive pixel peeping to discover and would then likely boil down to personal preferences as much as anything. I should also state that I do not give a rat's behind about high ISO performance. What I do care about is image quality of both stills and video and the GH4 more than makes the grade, FOR ME. YMMD.