Norton equivalent resistance of zero?
Your circuit for calculating the Norton resistance is correct.  The resistance between A and B is infinite since R4 is not connected to anything.  This actually makes perfect sense since your circuit, as far as points A and B is concerned,is just an ideal constant current source of 2 amps. The resistance of a constant current source is infinite. Any load connected between A and B is going to see only the constant current source.  The rest of the circuit is immaterial.  Thus your Norton equivalent circuit is a current source of 2 amps (in parallel with an infinite resistance).