\begin{align} \lim_{x \to \infty} x^4 \left( \frac{1}{x^2} - \sin \frac{1}{x^2} \right) &= \lim_{y \to +0} \frac{y - \sin y}{y^2} &\left( y = \frac{1}{x^2} \right) \\ &= \lim_{y \to +0} \frac{1 - \cos y}{2y} \\ &= \lim_{y \to +0} \frac{\sin y}{2} \\ &= 0 \end{align}