4. Let $X1, X2, \dots$ be independent identically distributed random variables with mean 0, variance 1, and $EX_1^4 < \infty$. Find the limiting distribution of

$$Zn = \sqrt{n} \frac{X1 X2 + X3 X4 + \dots + X{2n-1} X{2n}}{X1^2 + X2^2 + \dots + X{2n}^2}. \quad (25\%)$$