Hi, Diana!
Great question!
The reason you're hearing two different times is the following:
Mickey's Very Merry Christmas Party officially begins at 7:00 p.m. on select nights in Magic Kingdom. At that point, Guests without tickets to this special event will need to filter out of the park; they won't be allowed to board attractions after that time, either.
Similarly, in order to get the many Guests who have tickets to the party INTO the park, Cast Members may allow those MVMCP ticket holders to begin filtering into the park as early as 4:00 p.m. That earlier entrance time, however, is not a guarantee...but has been the pattern of entrance for the last several years.
For some great unofficial information about what to expect at MVMCP, head to one of my favorite sites, AllEars.net. In their search bar, enter Mickey's Very Merry Christmas Party, then click on the first link (most likely the post regarding the 2013 Parties). There, you'll find photos, videos and lots of specific information about what you'll find at the Party.
Have a BLAST, Diana!
Be sure to send more questions as you think of them, too; we love to help!