Do a KCL:
5A + 2Vx/2 = Vx/4. Therefore, Vx = -20/3
To solve for the open circuit output voltage, Vth, note that the current from the current source must pass to ground through the 4Ω and 6Ω resistors. Let V1 be the same as Vx and let V2 be the voltage at the top of the 6Ω resistor. Thus we can write:
V1/4 + V2/6 = 5