A Simple Question about outs

Hi, I have what seems to me a simple (and dumb) question, but I can't seem to figure out the solution so I figured I'd ask for some help. In Sklansky's hold em poker, the probability chart says there are 4 outs for a two pair draw/inside straight draw. What are the 4 outs (I think there should be only 3 if you're drawing to make a pair for your 2nd card or at least 5 if you include the opportunity to make a set w/ your other pair). I understand how there are 4 outs for the gut shot draw but I can't figure out how there are only 4 for the two pair draw. Thanks for any answers to my seemingly simple dilemma.