Of course, everything Disney is subject to change, but for the past 2 - 3 years they have been letting party goers in AFTER 4:00p with just their party tickets regardless of whether you have a PS or not.
They started doing this when they changed the party start time from 8:00 to 7:00 with only one hour to clear the park out (they used to close for 2 hours, from 6:00 to 8:00). They did this (letting people in AFTER 4:00p) so that not everyone is entering the park right at 7:00p so they won't be overwhelmed.
Anyway, I believe they will, indeed, let people in AFTER 4:00p again this year with just your party tickets and there are just a few CMs who don't know about this party procedure.
IMHO, I would change my PS to later than 4:00p if I didn't want to use my park hopper. Trying to get in BEFORE 4:00p without using a park hopper is really pushing it, IMHO. We are already getting an extra 3 hours (beyond the party hours of 7:00 - Midnight) to enjoy MK by them letting us in AFTER 4:00p.