I know this post is old, but why not resolve a question. From what I understand, you cannot record P60 or P120 4K on a GH4 internally or externally. Time will tell if there is a hack, but there probably won't be. And even then, I'm pretty sure there never was a higher frame rate hack for any of the GH cameras before. It's an awesome camera in itself and hacks are about as useless as jailbreaking is to iPhones now.
