need 4 gallons

You Need 4 Gallons!

        A ticking time bomb is attached to a weight scale that will disarm the bomb if you place on it a container with exactly 4 gallons of water.
    
     You have only two containers and lots of water to measure exactly 4 gallons.
    
     These containers A and B can contain 5 and 3 gallons respectively when filled, and there are no markings to indicate how much they contain.   

    Using the buttons you can fill, empty and transfer water between the containers.  To transfer water raise or lower the position of container B.

This puzzle was used in the movie "Die Hard 3" (1997)

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