Puzzle corner

The two jugs

     In on of John McTiernan's Die Hard ilms, Bruce Willis and Samuel L Jackson have to put 4kg on a scale in order to disarm a bomb, which is hidden in a fountain in a public park.
    To help them, they have two containers, one 5L and one 3L. (1L of water has a mass of 1kg)

Help them solve the problem before the bomb explodes!!!