Calculating Short Circuit Current when Nortonizing a Circuit
Guidances only:

1) Shorting the 8 Ohm resistor doesn't make the 4 amperes source redundant. 4 A is a part of the short circuit current. The rest comes from the controlled source.

2) Norton's theorem isn't a method how the equivalent should be calculated, Norton's theorem states that the equivalent current source with parallel resistor exists if the DC circuit is linear. It's possible that your teacher has declared "If I one day happen to order you to apply Norton's theorem to solve a certain voltage or current, then you should calculate the open circuit voltage and the short circuit current!". I do not believe such declaration really exists, but that's a belief.

3) when the circuit has controlled sources, you have a danger to use for ex. same i in different situations where the circuit is different due the needed opens and shorts to derive the equivalents. Your i is totally different when the circuit is the original, 8 Ohm is taken away or when 8 Ohm is replaced with a wire. 

I recommend you to stop with equivalent circuits to keep every controlled current and voltage in unique state. You get 2 equations with 2 unknowns i and v easily. Simply write i=10A+4i and the other equation for the currents at the right end of your 4i source.  With Norton's or Thevenin's equivalent sources - when applied right - you often generate only new unknowns to a circuit which has controlled sources and you will not advance at all.