W124 DTC 4, Air Injection Inoperable
 

1995 E420 148k

I've got a trouble code 4 (among a couple others) and if I understand it right, that means the "air injection [is] inoperable."

my question is, what do i do with that? looking around fastlane they have a part called the air inject check valve. does my code mean that part has failed? if so, where exactly is that puppy?

There is alot more to an AIR system than the check valve.
The first step is to see if the pump even comes on/runs..
Could be as simple as a fuse.