odds on turn card or odds on turn and river combined?

[ooloth]

when considering my chances after the flop and deciding whether or not to call, based on pot odds, should I think in terms of my odds of making my hand on the turn card, or on the turn and river combined?

for example, say I flop a gutshot straight draw to the nuts and I have no other outs. With 4 outs, my odds are 5.1 to 1 to make my hand by the river, so I figure I can call on the flop with pot odds 5.1 to 1 or better. On the turn, my odds of catching one of those outs becomes 10.5 to 1, so my pot odds will have to improve as well.

BUT, in their great book "Small Stakes Hold'em", Sklansky, Malmuth and Miller mention this situation and say that you would need pot odds of at least 10 to 1 to consider calling with 4 outs on the flop, because those are your odds of making your hand on the turn. I assume that these guys are correct, but I don't understand why you would want to think of your odds for just the turn card. Can somebody explain?