If I understand your question correctly, the answer is basically yes, you are correct. Suppose for instance that Laundry Detergent A is 4 times more effective at removing stains that Laundry Detergent B. That means that if Detergent B removes 20% of all stains, then Detergent A removes 80% of stains.
Hope that's clear.
Is it though?
Purely on a semantical point - and I'm not disagreeing with your answer, but if, say, a box of cornflakes contains 1kg of cornflakes, then a box that contains 4 times as many would contain 4kg, right? But would a box that contains 4 times more contain 5kg?
That is what I would say. "Four times as much" is 4x. "Four times more" is x+ 4x= 5x.
Of course, I have no idea which of those "factor 4 better/worse" is supposed to mean!
(Oh, and "4 more or less" is .)