Marble Bag

A bag contains $3$ red marbles, $4$ blue marbles, $6$ yellow marbles, and $6$ green marbles. Four marbles are randomly drawn from the bag, without replacement. The probability of drawing a red, then blue, then green, and finally another red marble is $\frac{a}{b}$ , where $a$ and $b$ are positive coprime integers. What is the value of $a+b$ ?