Hi,
Here’s a link to the ifixit [guide|86701] guide. The [product|IF300-058|LCD and midframe assembly] part is available from ifixit. Unfortunately the LCD only spare part is out of stock.
Here’s a link to the ifixit [guide|86226] guide. Unfortunately the ifixit part is out of stock but here’s a link to a [https://www.peauproductions.com/collections/gopro-hero-4-black-silver-lenses|supplier] of lenses for your camera. It is only shown to give an idea of the cost of the part. There may be other suppliers online that may suit you better. Just search for ''GoPro HERO4 Black lens part'' to get results.
Here’s a link to a [https://www.ebay.com/itm/Gopro-Hero-4-Battery-Door-Repair-Part-Action-Camera-Silver-A0903-/331857751121|supplier] of the battery door part. I couldn’t find one in black. You may have better luck. Again it is just shown to give an idea of the cost of the part.
Hopefully this is of some help.