DXOMark have tested the raw sensor output of the GH4. Raw stills have 12.8 stop dynamic range vs 11.7 stops on the 5D Mark III, illustrating just how far ahead almost everyone is on sensor performance over Canon.
Right now a lot of DSLR reviews mislead on high ISO noise. For example most recent Nikon D5100 reviews have a range of cropped colour chart samples shot under bright studio lights at different ISOs. Surprise, ISO 3200 looks great.
In real life it doesn’t!