Confusion over Norton equivalent problem
In the firs case, Ix current is 0A because 4 Ohms resistor is short (short across the 4\$\Omega\$)



And in the second case: 

 

As you can see \$I_{SC} = \frac{10\,V}{5\Omega}+2I_X\$ 

And \$I_X\$ current flow in the different path. Do not flow through node a and b.