Metabones BMCC Speed Booster apparently fits on the GH3 (and most likely GH4) for Nikon lenses. This would convert the GH4's 2.2 4K crop to a 1.4 which is slightly larger a Super 35mm crop (at 1.3)! BUT there's a rumor that Metabones is working on a GH4 specific Speed Booster so you may want to wait. What I really want is a Canon EF to M43 Speed Booster (like they have for many Sony mirrorless cameras) with full aperture and IS control. Now THAT would be sweet.
#sse
And it looks like using the Metabones BMCC Speed Booster on the GH4 comes with some caveats, mainly that it can hit the internal parts of the camera and possibly the sensor.
