How do I find the open circuit voltage of this circuit?
Can I just find the voltage across the current source and the voltage
  across the 4k resistor and add them together to get the open circuit
  voltage?


Yes, that's going to give you the right answer but finding those voltages requires a little thought. For instance you have to notice that the current through the 1kohm resistor is 9 mA and some of that comes via the 2k and some via the 3k so you have to proportion them appropriately AND keep the relevancy of the 45 volt source and 4kohm in mind.

It might be easier to solve it using superposition.