Probably around 2:30am!! If MK is open late, then fine..head there and be back at your resort no later than 1:30ish, in order to get any carryon bags out of storage (be sure to double check to make sure they are open at 2am!!). Then, be out at the bus stop about 15 mins before your appointed time.
If a park isn't open late that night, I would seriously consider staying at MCO that night...you can take a DME bus back there, stay the night and then head to the gate by 4. Much better than waking up around 1:30ish!