I have zero knowledge of what m43 sensor makers might be cooking up. I wouldn't be surprised to see a 20mp (5200x3900 or so) sensor in the next high-end models given that 20mp 1" sensors are already in use by Sony & Panasonic and apparently perform quite well. Pixel counts are most definitely not everything, though. The vast majority of camera owners have no need for 16mp, much less 20, much less 36, etc. Personally I'm looking forward to a 100mp sensor so I can process the files down to 25mp. I expect that'll be a future 35mm format offering, though. The greater real estate of larger formats ensures that smaller formats will never quite catch up mp-wise...unless of course a larger format falls out of favor and tech development for it declines or even ceases.
(BTW, the GH4 uses a 16mp sensor.)
-Dave-