Do you mean four times faster or do you mean four times as fast? These terms are not synonymous. Four times faster means the same thing as five times as fast.
i.e.: If it is four times as fast, you multiply the speed by four, and that's the new speed. If it is four times faster, you multiply the speed by four and add it to the original speed, because that's how much faster it is than the original.
I fucking hate that television commercials have conflated the two for the public at large. If you have followed in their footsteps, I hope this public service announcement has helped you sort that out for the future.
i.e.: If it is four times as fast, you multiply the speed by four, and that's the new speed. If it is four times faster, you multiply the speed by four and add it to the original speed, because that's how much faster it is than the original.
I fucking hate that television commercials have conflated the two for the public at large. If you have followed in their footsteps, I hope this public service announcement has helped you sort that out for the future.