If what I wrote wasn't clear I'll break it down.
Using VLC media player and the raw GoPro footage, extract the audio into a seperate audio file.
Or
Use Protune audio in the Hero5 Black (unfortunately can't be done with your Hero4 cameras) and let the camera create a second audio file. Some editors have issues with these files so you might need to put it in a free editor like audacity and then save the file in a new format like wav.
After doing the above, you can just mute the videos audio and insert the seperate audio file.
