Panasonic and Olympus have both heavily invested into the 4/3 and m4/3 systems, and even now they are still struggling to keep their head out of the water.
The format clearly saved Olympus from banrukpcy some years ago but nowadays where the mirrorless is getting more common and we have Sony landing with their A7 series, you almost don't have to compromise between camera size and sensor anymore.
The X-T1 (440g) is lighter than the OM-D E-M5 MkII (496g) and the A7RII (625g) is not that much heavier and has no compromise on the IQ. With every new advance of the larger sensor in the mirrorless world the 4/3 system dies a bit, the last bastion that holds them up is really that ocean of lenses, many of them are great primes.
