This is probably what happens.
The hero4 uses USB chip for communication and charging.
So upon connection to its USB port, the chip will investigate first, is this a PC trying to communicate or a charger only USB connection ?
The problem with power injecting thru USB is that you are using 5V, which is USB standard, so the USB chip has to do its work and reduce the voltage to approx 3.6V to 4V to power the electronics.
If you did what I did longtime ago with dummy battery, its a 3.7V direct to battery bay, so no issue.
Now, i done long recording using power bank and USB into the GoPro, no issue with wifi cut out.
What probably happened is this :
- Your USB connection is not very good or not a true 1.5 amps. If you have bad intermittent contact, that may explain the camera trying to recognize again and again "is this USB data or charging ?"
- If you don't have enough power, perhaps the wifi chip with uses 100mAh may not get enough power.
If the USB chip of Hero4 is the same spec as the older hero3/3+ , a low voltage and over voltage condition will also effect the camera.
- Your 5V is not a clean 5V. Perhaps too many ripple during the voltage reduction.
A 220V wall plug USB charger sometimes do that, dirty power.
As long as you can remind yourself that a USB is not a true charging port, you would understand why all these has to happen
meaning USB chip will do its work.
Now, with me able to do long record and operate wifi flawlessly using power bank USB to camera, I would suspect your 12V >>5V power kit is not really good/stable or clean.
Try the power bank method as verfication and let me know if that solves your problem.
Power ON the GoPro first and then connect USB power bank.