For an early flight such as this, I would book our last night at the airport Hyatt and get a couple extra hours of sleep. Wouldn't be missing anything at WDW at 3:30am.
Even during the wee hours of the morning the buses do come early. You need to be out at the bus stop, ready to go, 15 minutes before your scheduled time. My dad almost missed his bus back to the airport when I forgot to pass along that little nugget of information.