hello Gary i new wih hobby but i can tell you 3' pipe not enough it only carry 1800 gpm you need 5000 to 6000gpm due to 4000gl pond you have. it will be 1 bottom drain 4' about 3600 gpm and 2 skimmer 2' about 900gpm or one mid-water 3' about 1800gpm. i know it because i will have a pond like your pond
