Here is what I have done so far to diagnose a code4:

I checked the fuel pressure regulator vacuum and it is 20 in Hg. There is no fuel leaking out of it. I still need to check the fuel pressure but I think it is ok as the car idles and runs fine.

I checked the vacuum source for the AIR pump it is 20 in Hg. I checked the vacuum input to each one of the vacuum change over valves and they are both at 20 in Hg. I then check the vacuum during a cold start to the diverter valve and it is also 20 in Hg. So up to know I dont have a vacuum issue. The only two things left are the diverter valve and the air injection check valve.

How can I figure out which one is faulty? What should I do next?