I believe the GoPro Hero 4 records in the h.264 format which is contained inside the MP4 Format.
This codec is a Compression format and dependant on the recording bitrates.
h.264 is a universally accepted format that Allows for very large files to be compressed with minimal loss to the original content. The compression is dependant on the bitrate used; the higher the bitrate, the larger the file, and the lower the bit rate, the smaller the file. Satellite TV channels, although 1080p @24 FPS, are of a fairly low bitrate as they need to fit the bandwidth available. However, even then, sometimes images come across as blocky.
A Raw Image at full Bitrate will definitely be larger than the MP4 Image and taking into consideration, you can't have a video camera that can only take a few minutes of Film, the bitrate needs to be reduced to allow for longer footage.
If the Hero 4 is as good as the Hero 3, the loss of quality will not be something that will be noticeable and as good as if no compression had taken place.
Regarding editing, h.264/MP4 is pretty much the choice of all editing packages now. You may be required to install the latest version of Quicktime to ensure you have the latest codecs.