How can I figure out where power is coming from in 4-way switch wiring?
Cable 3 goes to switch A. 
Cable 4 goes to switch C. 
Cable 1 goes to the lamp. 
Hot is being supplied to the entire works by the one of the black wires at switch C. 
Neutral is supplied by cable #2. 


Supplying hot and neutral in separate locations like that is bizarre and not code legal.