Lots of ways to tackle this problem. Audio is a simple one to solve.
I typically use 2 cameras for my personal videos. One is recording generic cockpit audio (useful for listening to engine noise and other sounds) and the other is plugged in recording the intercom/radio sounds. This cable plugs in, in series with your headset, straight into the camera. THAT camera then has no sound on the video file other than the radio sounds. Erie quiet until you're talking... Once edited together, you get the full picture (so to speak) with regards to sound.
Here is the cable that I would use with your Hero 4.
https://www.nflightcam.com/collectio...io-power-cable
As for video, I usually will snap my fingers or something so I have a point to sync up the video between the two cameras during editing. For testing, I will have one either chest mounted (link) or suction cupped to the canopy where it can see the panel. The other camera, I'll have either head mounted or back on the turtle deck of my -4 for better situational awareness when watching the footage back.
This may be a good suction cup idea if you don't have one yet. The articulating arm may be handy for camera positioning but I'm not sure how it would hold up to vibrations in flight... There are other mount options that are more rigid but might limit camera positioning.
https://www.amazon.com/Flexible-Goos...5142703&sr=8-4
Here's two examples of how I utilize my cameras and edit it all together.
First is in the RV and I'm trying to demonstrate to GRT Tech Support the AHRS issues I have. No cockpit audio in this video but you can see how I use a wide angle (GoPro Fusion) for SA and smaller camera suctioned on the canopy (GoPro Hero 5 Session) in front of my face to capture the panel. Sometimes the suction cups don't like curved surfaces as seen in the video....
https://www.youtube.com/watch?v=zRETOgzs_B0
This next video in the DC-9 shows how I utilize both cameras for the audio experience. The upper camera (GoPro Fusion) is suctioned to a window above my head capturing airplane noise. The head mounted camera (GoPro Session) has the audio cable plugged in, in series, with my headset and captures just the intercom/radio audio. Note: This is a full flight from Laredo to Saltillo, Mexico so it's a bit long. Feel free to watch, but zip to about the 12:00 minute mark to see it all working together.
https://www.youtube.com/watch?v=249l6t9SSEo&t=1013s
I hope this helps!