just don't worry about the math of APS-C lenses. They are still fullframe lenses, but will have vignetting if placed on a fullframe camera.
Take for instace the Sigma 18-35 mm. If the speedbooster takes the crop factor of the GH4, down past 1.5x or below you will have vignettting @ 18mm. keep it at 1.6 and above, and you're good.