There will be at least one wristband distribution point open inside the park every night the party is offered. Once 4 PM hits (or close to that) they will have you scan your party ticket and they will put the wristband on you there. No need to exit the park and enter again.
Those entering through the front of the park after 4 (with just a party ticket) will get their wristband there at the gate immediately after scanning their ticket.