Traveling with little ones can be a challenge but especially so if you might get stuck somewhere. Don't worry about packing too many diapers - no such thing! If you get stuck, you'll need them! Powdered formula doesn't take up much room so take extra. Again, you don't want to run out. Besides, what you don't use on the way down you can re-pack for the trip back. Just estimate what you'd use for a couple of days plus the plane ride and bring that. Pack at least two FULL changes of clothes. Diapers have a way... Be sure to pack plastic grocery bags or smaller trash bags for wet/smelly clothing (it happens). Bibs are VERY useful. A light to med weight easy to put on/take off sweater/blanket is also good on the plane and in the terminal if you have to wait.
One trick - leave the heavy coats in the car. You won't need them in FL. When you return, one of you goes out to get them. Take a couple of light-weight jackets for each of you. Layering is great - a shell and a sweatshirt type give you options both on the plane and in FL.
As far as keeping baby calm during the flight, she takes her cues from you. If you are happy and relaxed, play games, make the bumps and noises fun, she'll be fine.
A well-designed diaper bag can be like Mary Poppins magic carpet bag. Favorite toys/books (small) brought out ONE AT A TIME are good entertainment and snacks/bottle (if they're allowed yet) given during take-offs and landings help with ear pressure changes. Sugar-free cereal like Cheerios work great (maybe not for one her age but that's up to you). Pack several small zip-loc bags vs. one or two big ones. They're easier to handle in small spaces and a lot less to pick up if one spills. Pacifiers, too. Encourage that chewing/sucking/swallowing to pop the ears! Keep all of you well hydrated (water/juice, not pop). Airplane air is VERY drying. Speaking of air, be aware of where the air vents are blowing. The force changes greatly during pressurization.
If you can get seats at the bulkhead, diaper changes are a LOT easier and QUICKER because you have room to move (a wipeable changing pad on the floor instead of a blanket is advised). Do NOT get the row in front of the exit rows (if I remember right, the seats don't lean back as far).
Baby in carseat in plane seat vs. baby in lap. If possible, go with the car seat in the plane seat. She's used to it, already sleeps in it, and she can be buckled in for safety if there happens to be a little bumpy weather (if it does get bumpy, stay happy and upbeat even if you're not - they can tell if you're worried and will react). You can move around as needed and you'll be able to use your lap tray (staging area for snacks/drinks/toys rather than tucked in the seat around you). Also, body to body gets VERY warm after 4 hours. Even after one. If you are doing the lap route, trade off with your husband - frequently unless she's asleep. New lap, different toy, different game. Check ahead of time to see if the carseat can go in the luggage area near the galley (can't remember its actual name). They rarely fit in the overhead.
The main thing is to have fun and make it an adventure - even at the young age of four months. If it's a pleasant trip going down, it should also be fine coming back. I know our two girls enjoyed travelling and they each started at about the same age as yours. Over four trips from Rochester/Mpls to Orlando or West Palm Beach and all the ups and downs that entailed (then the car ride to Ft. Pierce - another good reason to have the car seat) and back, the flight attendants often commented on how good they were. Relax and enjoy your trip!