Update: She took it to a shop today, and they verified that it is indeed running on only 3 cylinders, they pulled the #4 plug wire and observed no change to idle. They also observed a good strong spark there.
So I guess the next step is to pull the injector connector and ohm out the injector. If it ohms out good, I would guess there is a faulty connection to the injector. That would be super cheap to fix if it is easy to find and get to. If it's the injector itself I still have to imagine that would be a bunch cheaper than a new cat. My recollection of pulling injectors is that they can be a PITA and need a flare wrench depending on how they're oriented and what's in the way of getting a socket over them.
Thanks again for all the helpful suggestions!