It definitely sounds like overactive letdown, and an oversupply. What length blocks are you doing? You might try a longer block, as long as he continues to gain weight nicely. Keep in mind, too, that at four weeks, your supply hasn't regulated itself to the demand yet. That usually happens around six weeks, which also happens to be the peak of infant fussiness. So it may be that you just need a little more time, and the supply will settle down all on its own.
However, I think that with mucus in the stool, I would be concerned about dairy protein (casein) intolerance. Mucus is usually a sign of intestinal injury-- from a virus, from a dietary intolerance, from a disease. The overactive letdown symptoms usually show up as green, foamy poo. It's possible, of course, that it's all related to that. But I think I would be watching it carefully, and if reducing the supply doesn't get rid of the mucus, I would be thinking about an elimination diet. My DS was casein-intolerant. It first showed up as green poop, with mucus sometimes. Anyway, it progressed to the point where I started seeing blood in his stool, before I realized what happened, and by then he was miserable. I wish I'd realized sooner what was going on.
