4.
已知\({{F}_{1}},{{F}_{2}}\)是椭圆\(C:\dfrac{{{x}^{2}}}{{{a}^{2}}}+\dfrac{{{y}^{2}}}{{{b}^{2}}}=1(a > b > 0)\)的两个焦点,\(P\)为椭圆\(C\)上一点,且 \(\overrightarrow{P{{F}_{1}}}\bot \overrightarrow{P{{F}_{2}}}\)\(.\)若\(\Delta P{{F}_{1}}{{F}_{2}}\)的面积为\(9\),则\(b\)\(=\)____________。