They treat it as two separate events. You are permitted to enter the park at opening and stay until 7pm (I think), at which time they begin to usher people out along one side of Main Street as others who are entering for the party but missed the early 4pm admittance, are entering up the other side.
If you were already using a regular park ticket, then you can hop to another park. The advice I usually give to people is, unless you were forced into buying length-of-stay tickets, if you are planning on attending the Halloween party, do NOT waste money on going to a park that day because it's technically only valid until 4pm. I never really saw the point of using up a fully day ticket and then only using it for half day. Spend some time at your resort, swim, shop, whatever, but don't bother with a park on the same day. It just doesn't make sense.