We take off the protective casings of our GoPro cameras to get better sound and video under low light. We bought a mount and lens cap from eBay so we can mount the cameras on stands.
But... unfortunately the lens cap was dodgy - and it scratched our Lens after a week of use! Arrgg...
So we bought a replacement lens from eBay again (fingers crossed) and replaced the lens.
This is a great guide to follow when replacing your GoPro Hero4 lens...
https://www.ifixit.com/Guide/GoPro+Hero4+Silver+Lens+Replacement/50702
If you are too cheap to buy a good replacement lens, it is also possible to sand and polish scratches out. Here is a guide to do so...