3- the book it's in
9- the number of letters in Snape's nickname given by the marauders. s-n-i-v-e-l-l-u-s
4- the number of tortures he had- Sirius, James, voldemort, his love for Lilly, or if you don't think of it that way, the four marauders.
It's just the way he says it. Normally with a supply teacher you expect them to let you know why your original teacher isn't with you, but with Severus all he does is walk in like it's his usual lesson and says, "turn to page 394". And everyone follows! :)