Welcome, re quad I'd consider a few things if you are riding anywhere rather than private land with the owners permission you will need some form of Insurance, bike will need to be registered. Stick to private land, plod can seize your new bike otherwise. Re kit you will need to go somewhere where you can try it on as he's a young lad. I'm sure there must be clubs out there which would help as you need to make sure of safety ideally some sort of Marshalled course with first aid provisions. Personally 4 seems too young but maybe do some local research to keep safe if your gonna do that