Angela,
Thanks for asking your question to planDisney!
No, you do not have to leave the
Magic Kingdom Park and re-enter if you are also going to
Mickey’s Very Merry Christmas Party that same day. As a matter of fact, that special event ticket entitles you to enter the park starting at 4 pm, not that it will matter to you.
You will already be there in the magic! So, you are good to stay and enjoy the fun.
I am actually doing the same thing later this year. Disney cast members will check that you have that valid event ticket and then let you continue your day in the park without having to leave.
BONUS TIP
Guests to Mickey's Very Merry Christmas Party enjoy the shorter attraction lines that the smaller event crowd affords them during the party. You should as well, of course. But don't overlook the opportunity to slow down and enjoy the holiday ambiance of the park. A stroll down Main Street U.S.A., decked out for the season, is a magical experience that you will want to savor. Grab a hot chocolate and bask in the glow of the park.
Write me back if you have any follow-up questions.
Have a Magical (and Merry) Vacation!
Doug