I have a Rheem powervent 84 gallon tank. The control valve on the unit has an LED indicator light that is flashing 4 times. The code says that if it is flashing 4 times then the pressure switch is open. What does this mean, and how do I fix this. I have no hot water.

A switch can be open or closed. If closed, current flows through the switch to whatever it's supposed to go to. In your case, the pressure switch is open -- probably failed open. That means that you need to replace it with a new pressure switch.