gopro hero vs gopro hero4 black video quality diference
There are charts on their website, but in short, the Black allows for 4k recording, 60 mbps bandwidth, higher frame rates at various resolutions, simply put, it has a larger bottleneck enabling it to capture more data. 

I would suspect the cameras may be perhaps identical, and are firmware locked down into lower levels for the less expensive versions. Bypassing that would be, well very difficult if not impossible. 

The Blacks are great, I own a handful and use them all the time.