Justin is so confusing when it comes to dating. He could date someone older, he could date someone younger, he could date a giraffe. So yes, he could date someone four years younger when the partner is legal. (:
Everyone could disapprove because he’s now 20 and she’s (almost) 16, but it’s not illegal. If they did anything sexual, then it will be illegal. Justin would probably try to persuade them, trying to tell them that their relationship won’t go any further until they’re both legal and ready. But through all the shit he could get, I see him still staying with her.