It's advertised as a regulator, but believe it is probably actually a defective shut off as you described.
Obviously it cannot handle 4500 psi. The question that needs to be answered is if it regulates the pressure thereby controlling the air flow, or if it only regulates the flow by increasing/decreasing the size hole? If it controls pressure, that is what you want. If it controls air flow, you will need to have a pressure regulator before this; otherwise everytime the flow stops, the pressure increases to its maximum. If you have 90 psi in the tank, when spraying, it will start at 90 psi pressure, then drop to some lower pressure the longer you spray. I ran into this problem trying to regulate a spray gun by buying its equivalent product at HF.