Yep, I enhanced it, it's ductwork (that should be re-done also from the look of it) Man, what a mess you have there. If you really want to do it right (and you should) it pretty much all needs to go. I'll bet the only vent in the house is the 4" stack. big job, lots of problems. some could be structural. If you want it done right and not have to worry about it you should probably call a plumber in and get his thoughts.