Question about adding FP

We have 4 day tickets with park hoppers. Am I only allowed to book FP for 4 different days, or could I book them for each of the 5 days we will be there and decide later which day to cancel?

We may add a day of tickets but I haven’t decided yet. I am also unsure which of our days would be our resort day, so hoping to book our FP for all 5 days and then once I decide which day we aren’t doing a park, I would cancel them.

Advice?

You are only allowed to book Fastpasses for the number of days that you have tickets for. Did you buy the tickets from Disney? If so, you should be able to change the number of days online. But if you bought from a 3rd party vendor, you will have to wait until you arrive to add a day and book FP for that day.

Thanks, got them through Disney. I am considering adding an additional park day, so I may just go ahead and do that.