Effective odds

[countrynole]

Ok, I started reading Sklansky's book "The Theory of Poker," and I got to the chapter about effective odds. The example that he uses is getting a four-flush off the flop in hold 'em. He then says that the odds of completing the flush within the next two cards is 1.75 to 1; and if you dont get it on fourth street then the odds are about 4 to 1. My question is, how did he calculate the odds of getting that card on fourth street?

P.S. I don't really need an explanation, just an equation would be fine.

Thanks