Any chance it's the banjo bolt copper washers at the outlet of the pump head, where the return hose/pipe assembly is attached?
Between the washer under the check valve and the pump, and the two bracketing the pipe, there are 6 sealing surfaces that could possibly leak, if there was a flaw in the surface or how they were installed.
They're relatively cheap, and easy to replace if you want to test that.
Another possible diagnostic aid would be to put a length of clear hose between the #4 injector and the return pipe at the pump, to see if the bubbles are coming from the injectors or at the pump. I would expect that some basic 1/8" small engine fuel hose would work, at least for a short period of time.
Between the washer under the check valve and the pump, and the two bracketing the pipe, there are 6 sealing surfaces that could possibly leak, if there was a flaw in the surface or how they were installed.
They're relatively cheap, and easy to replace if you want to test that.
Another possible diagnostic aid would be to put a length of clear hose between the #4 injector and the return pipe at the pump, to see if the bubbles are coming from the injectors or at the pump. I would expect that some basic 1/8" small engine fuel hose would work, at least for a short period of time.