You have to start the car with the hood open..it is the belt driven pump over by the air-filter ... you can hear it run.. It only runs when the engine is cold at start-up and it only run for a min or so.
It is emmissions device that pumps air into the exhaust just til the cat heats up. Usually when that code comes up, my firsts check [ after I know it is actually coming ON ] is to pull that plastic cover in front of the engine..you will then see a white plastic vac line crossing over to a couple of valves ..that line always cracks and breaks and when it does , the Air Pump valve does not work so that code comes up as #4
I would also look at all those lines in that area seeing thatyou just worked in that space. May just have a vac line off.

Check those out and see if you find anything..