4K @ 60fps is too much for the internal processors to handle. It has nothing to do with the recording media, the camera simply can't process that much data onboard... for now.
You can't shoot 120fps using the 35mm sensor size because the sensor can't scan the full surface area at 120Hz. It can only do the s16mm sensor area at the full 120Hz. Once again that's too much data to handle internally by the onboard processors, but it can cope with a down-rez and encode to RGB. So we should be able to get 720p 4:4:4 RGB at 120fps onboard. We should be able to get 720p and 1080p @ 60fps from the s35mm sensor area at 4:4:4 RGB onboard. ...Once all those RGB functions are enabled.