I think it depends on how completely the gearbox gets drained. 4 quarts did my gearbox with a bit to spare, but I think if the gearbox is bone dry then over 4.5 quarts will be needed on hand to ensure it gets topped off.
But one definitely won't do the job! Plus, be aware that Heavy Shockproof is extremely thick stuff and it doesn't pour. Well, it pours, the same way paint dries. So some sort of heavy-duty pump to get the stuff in will be needed, unless the gearbox is out of the car (in which case you can then take your time dribbling in all four bottles worth).