Obviously you did not reach the level when you can program the behaviour of four elevators in the same time :) About the first and the 20th floor: How about if

Obviously you did not reach the level when you can program the behaviour of four elevators in the same time :) About the first and the 20th floor: How about if you've got 15 people on the 20th floor and one person on the 1st? Every button press is a request that could add more weight to the particular floor. After all the goal is to transport as many people as possible for a given time.