Probability of 4 dices

Four fair six-sided dice are rolled. Let $x_i$ $(1 \le x_i \le 6)$ denote the number of dots on the top face of the $i$-th die $(i = 1, 2, 3, 4)$. The probability that the four numbers $x_1, x_2, x_3, x_4$ can be divided into two groups having equal sums is $\dfrac{a}{b}$, where $a$ and $b$ are coprime positive integers. Find $a + b$.