My husband and I have traveled with my four year old a few times on longer vacations to Florida from Boston and found some success.  First I would highly reccommend traveling on a direct flight that is very compatible with your child's mood and/or nap time.  Even though the flights that make stops might be less expensive, I wouldn't do it.  Also, I would get a separate seat for my child.  Before my trip I would go to the local Walgreens or any other places you know to buy cheap toys.  I would stock up on puzzles,, books, crayons and coloring books, small magnadoodle, playdough, or travel size lite brite.  Anything you can find to help occupy your child.  When buying the toys for my carryon I would buy extra and pack them in my suitcase for the ride home.  This makes both directions of the trip filled with new toys that will occupy them a little longer than the old standbuys.  Another good thing to pack is snacks.  With the airlines not offering too many goodies nowadays I would be prepared with my own know favorites.  Also, because of the security restictions, airlines are asking people to arrive very early for scheduled flights.  I wouldn't arrive too early, because the earlier you arrive, the longer the time you have to occupy your little one(s).  So, leave enogh time to get through security and have about 20 minutes to spare for the flight.