[AaronBrown]

You've figured the probability of the four specific pocket pairs (J, T, 8 and 3) rather than just four different pocket pairs. I'm not sure which one was meant by the question. Second, you've figured the probability that one specific one is quads (the 3 in this case) rather than just that one gets quads. Third, you have to multiply by 2/5 to get the probability that the quad shows up on the river.

But the trouble with calculations like this is any had is wildly unlikely after it is dealt. I admit this is an unusual hand, but it's not as unusual as the numbers seem to indicate. If you watch a few thousand poker hands, you will see one with four players with strong hands and an innocent looking board.