There is a little bit of a trick to this.
Lets say you are going to 5 days (M-F) and one day is a party day. So your family has decided to buy a 4-day ticket, and use your party ticket as your park ticket for that day, with plans of entering the park as early as 4pm. So lets say M, T, W, F are normal park days, and TH is the party day.
When making your FP+ selections, you have to choose your FP+s your party day LAST. If you don't,
MDE will use one of your 4-day ticket days that day, and you will be unable to make FP+s on the last day you want to use your 4-day ticket. So you will want to make FP+s for M, T, W & F, leaving your TH plans for last.
After making the FP+s for your 4-day ticket, when you go in to make your FP+s for the day of the party (TH in this scenario), you will be limited to only making FP+ reservations after 4pm. Then you can make FP+ reservations anytime between 4-7pm on that day. After 7pm there will be no FP+ and every ride will be stand-by.