At this point would do a compression check on number four to see if it is the block or injection pump gone bad. About the easiest way to divide it up. A quick and dirty test of the pump would be to hook an injector up to a line from #4 element of the injection pump and see if it is spraying with the engine cranking. Do you have a spare injector line around? Perhaps the present line could be oriented to perform the test? You might also want to have a look topside to see if you broke a valve spring on #4 or any other irregularity on that cylinders valve setup. I would only do that if the pump passed the spray test well. Your observations almost tend to indicate the #4 pump element has gone bad though. Visually more fuel out of the first three injector lines you mentioned. There possibly is a check valve on the fourth element to stop fuel backing from the line under pressure and it may have failed. I leave it to others with more experience to comment.