Hi, Rebecca! In years past, just like this year, the official party time begins at 7:00 PM. They have always allowed guests into the theme park at 4:00 PM with their party ticket, which allowed for time to make some dinner reservations, buy party memorabilia (pins and t-shirts), or even hit some of the shops to put the final touches on your costume.
Guests were directed to a specific set of turnstiles only used for guests of Mickey's Not So Scary Halloween Party. There, you were given your party wristband and a trick or treat bag, and maps for the evening's festivities were available.
There are quite a few of us Moms who have made inquiries while we were in Walt Disney World over the past few weeks, and even more of us who have made phone calls, and we have been told by most of the Cast Members that guests will still be permitted to enter at 4:00 PM, especially if they have dining reservations in that time frame. I will say, however, that the official time to enter is still noted at 7:00 PM.
Please feel free to check back with us as we get closer to the dates of the party. Hopefully, we will be able to provide you with more concrete information at that time!