A:AnswerIt really depends on the 4 year old. I would say no if ot was their first console game. Yes if they've played other games.
They can probably get through the first few levels, but need help the rest of the way. You can play 2 player co op, so you can help them get through the harder parts.
A:AnswerI don’t think there is an exact reason for it. It’s always been like that for many games. Even movies you can purchase as digital seem to still be pricey. I personally just got with physical.