Name the empty trucks truck1, truck2, truck3, etc. Name your trigger. In my example it's called myTrigger.
CONDITION: truck1 distance myTrigger < 50 AND truck2 distance myTrigger < 50 AND truck3 distance myTrigger < 50 AND truck4 distance myTrigger < 50
ON ACTIVATION: Whatever you want to happen when the trucks reach their destination