Ok, you want to create a air only location to keep the dropship still, let's call it 'dropship'. Just always move the dropship there, then order the 4 units you want to use into the dropship (probably 4 tall units ... like zell, dt, vulture, hydra).
Then you have triggers that check to see if all 4 units are in the location 'dropship'. If any unit is NOT in that location, that will be the condition to your trigger, and the actions will be to order a unit to move the direction you have assigned to that unit. You will then order the unit back into the dropship via the "Enter transport at location" AI script.
If you don't understand that, you could always refer to the tutorial made for dropship events.
http://www.staredit.net/index.php?act=tuto...asting%20System