After what felt like decades of asking for this product, Metabones finally has released a Canon EF Lens Speedbooster for Micro Four Thirds cameras like the Panasonic GH4. The Canon EF Speed booster will increase maximum aperture by 1 stop and modifies the lens view to 0.71x wider. The adapter can be powered via 5V through a MicroUSB port for the built-in electronics to change aperture values.
Unlike the RedRock Micro LiveLens Adapter which can adapt a Canon EF Lens to MFT Cameras, the aperture value has to be changed 'within the adapter'. The Metabones is a smart adapter that communicates to the lens so that aperture controls can be dialed in right from the camera as if it were using a native MFT lens. Apparently it is shipping now, and you can find out more at the Metabones website (click here).
Metabones Canon EF Lens to Micro Four Thirds Speed Booster