Just received my EF - M43 Speedbooster and I cant check it out with my GH4 as that's currently out on a shoot , but I did check it with the Black Magic Pocket CInema Camera BMPCC.
the Fit is REALLY tight on the BMPCC. Tried it with the 70-200mm F/2.8 II and its great. even at 200mm I can hold it handheld and get really decent stabilisation. Turn IS off and its a dreadfull.
Will be another few days before my GH4 is returned so will be keen to see what the fit is like onto that.
Can anyone tell me what the other little lever is on the side of the speedbooster ?