Which is correct about the cap of Paladin Divine Smite?

@Knoll68028721Which is correct about the cap of Paladin Divine Smite? 4th-level spell slot (5d8) or 6th-level spell slot (2d8 + 5d8)? 5d8 + 1d8 if the target is an undead or a fiend.

— Jeremy Crawford (@JeremyECrawford) February 3, 2016