Nonlinear Systems & Linearization · Easy problem

For the oscillator $x' = y,\; y' = -x^3$, the energy $E = \dfrac{y^2}{2} + \dfrac{x^4}{4}$ is conserved. A solution starts at rest at $x = 2$, i.e. at $(2, 0)$. What is the largest speed $|y|$ it ever reaches?