Seems your thread got slightly highjacked.
I have the 4 and it is a great camera, easy to use and does good quality video, I use it to supplement my full camera setup, especially when I don't have a wide angle lens on my main camera.
DJI are massive compared to GoPro, GoPro only do action cameras (and they massively failed when they tried to release a competing drone to DJI, and ended up recalling them all and discontinuing there drone development)
Whereas DJI do multiple drone models from small to very large heavy lift drones, audio recording products and also do the Ronin series of professional stabilised video products with there own sensors and optics (that a lot of movies are shot with for there action scenes)
The also own part of Hasselblad that produce quality camera optics.
Their support has become very good and responsive.
The main reason I moved from GoPro to DJI was that GoPro has a history of overheating and locking up needing the battery to be removed to recover the camera (and something you can't do underwater)
I can't speak much about the difference between the 3 and 4 in real life as I don't have a 3, but there were improvements in the sensor, stabilisation and storage size.
I did have one issue with the Action 4 where I used a Samsung SD card that turns out isn't compatible, swapped it over for a SanDisk card and problem solved.