Downvoting doesn't cost karma directly. The phrasing was a bit off. What they mean is the following: There's a maximum limit to the number of downvotes you can have. That limit is 4 * karma. But, votes on top-level posts are amplified by a factor for for total karma and for how much they count towards the 4 * karma limit.

Edit: How does one do *s without triggering italics? Edit: And thanks very much for that.