You would be surprised about the high ISO performance of the last micro 4/3 cameras. This is a shot at 5000 ISO (No noise reduction applied). Sure, this is not the ISO performances of the new D750 but it is as good or even better than what my 5D Mark II was outputting. This was taken with the Panasonic GH4.
Micro 4/3 are great sensors but there is one physical fact and that's a FF sensor collects 4 times more light then micro 4/3 and there is no way to go around that so getting a micro 4/3 is a compromise compared to FF which has its drawback like much bigger and heavier overall system....MUCH!!!
If weight and size is an issue then micro 4/3 is an excellent way to go but if you like me and you willing to "schlep" your equipment then FF is an obvious winner for stills photography.
Where did you get that idea of 4 times more light for a FF sensor? If it gets 4 times more light with the same scene, it means the image is overexposed by 4 stops
It does not work like that in real life. In real life an APS-C and even more so 4/3 camera can use a wider aperture for the same shot, simply because of a deeper DoF. Which allows for lower ISO than FF for the same shot.
With a FF camera you often need to step down the aperture, because the DoF is too shallow for many shots and then crack up the ISO to compensate for that Yes, high ISO performance is great with FF, but 4/3 sensors do not need that ISO for the same shot in the first place.
So there is an obvious way around that.

As I said FF high ISO is often overrated .