Hey, Marc!
Welcome to the Disney Parks Moms Panel! Looks like you have an amazing Disney adventure on the horizon.
You do not need both tickets to enter Magic Kingdom Park for
Mickey's Not-So-Scary Halloween Party. Typically, you will be allowed in Magic Kingdom Park at 4:00 p.m. with the party ticket giving you an additional 3 hours of magic to enjoy before the party begins at 7:00 p.m. If you want to attend Magic Kingdom Park from open till the end of the party, then you will need both tickets.
I would plan my trip just as you have described with a
4-Day theme park ticket and the Mickey's Not-So-Scary Halloween Party ticket as an additional 5th day.
Disney Dad Tip: I would suggest using the hours prior to the party to enjoy your resort or some miniature golf at either Disney's
Fantasia Gardens and Fairways or
Winter Summerland Miniature Golf courses. You could also head to
Disney Springs for some great eats and shopping.
Marc, enjoy making many Magical Memories.
~Jason ºOº