[gaming_mouse]

This should be easy.

First, how many flops contain quads? Well, two of the cards are already determined. That only leaves the final card, which can be any of the remaining 48 cards in the deck.

So 48/(50 choose 3) is the chance of quads on the flop.

On the turn we can make our quads 48*47 possible ways, since we have 2 free cards which may vary.

So 48*47/(50 choose 4) is the chance of quads by the turn.

River is similar.

gm