Quad-boob occurs when a woman's bra is snug enough that part of the breasts squeeze up above the fabric of the cups, creating an upwelling of boobs visible even with a shirt on top. To the unlearned boob admirer it may give the impression of four boobies.
Check out that quad-boob over there, she must have escaped from Jabba's Palace.
When a woman's breasts spill over the top of her bra, giving her the illusion of having four breasts.
I'd better go up a cup size because all my bras are giving me quad-boob.