Look at it from the pure numbers and it's far less insignificant.
3 million x50 (Super Saiyan) = 150 million
3 million x400 (Super Saiyan 3) = 1.2 billion
3 million x500 (Super Saiyan 4) = 1.5 billion
It's a difference of 300 million, and even higher with a stronger base.
Plus Super Saiyan 4 has less stamina loss than any other form.