Hi Jennifer! My own children often have this dilemma when planning their Walt Disney World vacation. I will share with you what I tell them.
You are correct in wondering if flights will be delayed and since you will be coming from a northern state where snow is often in the forecast it could easily happen. My suggestion is to use fifth day of tickets for the last morning. I am assuming from what you said that you won't have to leave Disney until 2pm which would give you plenty of time to head to a park, take in three
FastPass attractions, and also grab a bite to eat. Be sure and give yourself about an hour travel time back to your resort to retrieve your luggage that you can store with the bell staff and take transportation to the airport.
I suggest you consider taking in Disney Springs for your first day. It is such a wonderful place to walk around, shop, and dine at one of many great quick service or table service locations. There is always something to see and do there and you can use Disney buses to go to and from your resort.
Hope this helps and enjoy your upcoming vacation!