4. As shown in Figure 3, in the isosceles right triangle $\triangle ABC\left(\angle C=90^{\circ}\right)$, take a point $P$ inside, and $AP=AC=$ $a, BP=CP=b(a>b)$. Then $\frac{a^{2}+b^{2}}{a^{2}-b^{2}}=$ $\qquad$