I don't see anything on Disney's website for the party mentioning early entry. But I know people get e-mails mentioning 4pm.
From the various comments I got the impression that it's a grey area on the early entry. It's def. not openly advertised like Universal does for Early Entry for Harry Potter if you stay at their hotels.
I think there is sorta a confusion thought because people get e-mails after they purchase tickets saying they can book FPs up to 60days if onsite or 30days if offsite but if you go to the actual page on WDW's website it says the following:
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The special event hours are from 7pm-midnight. So the confusion almost appears to be the way it's worded in the e-mails because Special Events are separate from normal park admission. Just like with EMM you still need to purchase a park ticket on top of EMM ticket for that special event.
As far as I can tell it's not specifically advertised that you get FP privilages from 4-6pm just with a MNSSHP ticket but that has been what the norm was for a while-almost like Disney wasn't paying attention to that. Who really knows to be honest that's why I feel like it's become a grey area---perhaps Disney is now making it more a black or white area..I really don't know.