Say we have four points $(a, f(a)),(b, f(b)),(c, f(c)),(d, f(d))$ on the curve which form a rectangle. If we interpolate a cubic through these points, that cubic will be symmetric around the center of the rectangle. But the unique cubic through the four points is $f(x)$, and $f(x)$ has only one point of symmetry, the point $(0,0)$ So every rectangle with all four points on $f(x)$ is of the form $(a, f(a)),(b, f(b)),(-a, f(-a)),(-b, f(-b))$, and without loss of generality we let $a, b>0$. Then for any choice of $a$ and $b$ these points form a parallelogram, which is a rectangle if and only if the distance from $(a, f(a))$ to $(0,0)$ is equal to the distance from $(b, f(b))$ to $(0,0)$. Let $g(x)=x^{2}+(f(x))^{2}=4 x^{6}-8 x^{4}+5 x^{2}$, and consider $g(x)$ restricted to $x \geq 0$. We are looking for all the values of $a$ such that $g(x)=g(a)$ has solutions other than $a$. Note that $g(x)=h\left(x^{2}\right)$ where $h(x)=4 x^{3}-8 x^{2}+5 x$. This polynomial $h(x)$ has a relative maximum of 1 at $x=\frac{1}{2}$ and a relative minimum of $25 / 27$ at $x=\frac{5}{6}$. Thus the polynomial $h(x)-h(1 / 2)$ has the double root $1 / 2$ and factors as $(4 x^{2}-4 x+1)(x-1)$, the largest possible value of $a^{2}$ for which $h\left(x^{2}\right)=h\left(a^{2}\right)$ is $a^{2}=1$, or $a=1$. The smallest such value is that which evaluates to $25 / 27$ other than $5 / 6$, which is similarly found to be $a^{2}=1 / 3$, or $a=\frac{\sqrt{3}}{3}$. Thus, for $a$ in the range $\frac{\sqrt{3}}{3} \leq a \leq 1$ the equation $g(x)=g(a)$ has nontrivial solutions and hence an inscribed rectangle exists.