Phil - If you have two of the quads in your hand, that leaves two unaccounted for cards, and you need exactly both of them, c(2,2) times the unknown card in the third slot c(46,1). I don't understand your method, and I especially don't understand where the c(4,1) number comes from.
Edit - I just checked wikipedia, and it states that if you've got two pair, the odds of flopping quads is 0.0053191, which is pretty much exactly twice the number I posted. Beer me, my friend.
http://en.wikipedia.org/wiki/Poker_p...ing_high_hands