Yes, CP does take reservations continuously throughout the day, so you would be able to get one for 4:30 (depending on availability--it's pretty popular for the parties).
Disney doesn't actually usher everyone out of the park for the party; they just issue wristbands to everyone who has purchased a ticket. Once it hits 7:00, no one without a wristband will be able to go on any of the attractions or trick-or-treat at any of the stations. They also seem to set up CM "roadblocks" where they'll stop non-banded people from entering any of the lands from the central hub. I've heard that they also patrol the parade crowds checking wristbands, but have never seen that.