Smatree 1290mAh Replacement Battery (3-Pack), 3-Channel Charger with USB Cord for Gopro Hero 4 Camera (Only for GoPro HERO4): Amazon.co.uk: Electronics & Photo
GoPro Hero4 Session Teardown - iFixit
Easy way to take apart (or destroy) a GoPro Session | Part 1: Getting the case off - YouTube
Replacement Internal GoPro Hero 4 Battery 1160mAh HERO4 Battery Silver Black UK | eBay