Imax = \frac{\pi }{4\sqrt{2}} ???
I min = \pi /6??
x^{3} \leq x^{2}\; for\; x\;\in [0,1]
so\; 4-x^{2}-x^{3}\leq 4 - 2x^{2}
so\; \sqrt{4-x^{2}-x^{3} }\leq \sqrt{4 - 2x^{2} }
so\; \frac{1}{\sqrt{4-x^{2}-x^{3} } }\geq \frac{1}{\sqrt{4 - 2x^{2}} }
so\; \int_{0}^{1}{\frac{1}{\sqrt{4-x^{2}-x^{3} } } }\geq \int_{0}^{1}{\frac{1}{\sqrt{4 - 2x^{2}} } }
similarly use x^{3}\geq 0 \;for \;x\in [0,1]
so\; 4-x^{2}-x^{3} \geq 4-x^{2}