As it is you will be able to enter at 4 PM which is 3 hours before the start of the party. If you are entering the MK using only the party ticket you will have to wait until the turnstiles are enabled for the party tickets. It might be a little before 4 or it might be at exactly 4 PM, but there's no way of knowing. For a dining reservation you need to allow time to enter the park, get your wristband and trick or treat bag and walk to the restaurant so it is best to make a reservation no earlier than 4:20.