....u dont. but it plays normally so i never questioned it. bridge takes the positive from one and negative from another chan. i would assume that if the 4th chan negative was weaker it would create uneven sound waves of opposite phase and sound like crap. but it seems normal. so i guess luckily there is sumtn wrong with the single polar output terminal that is not used ont he 4th chan while bridged. which is was makes this amp a very nice 3 chan. bout 75x2 and 150x1 . the loss of the 3rd chan made me drop price from 300 to 200