Rewrite $\frac{1}{x^{-4}}$ using a positive exponent, where $x\ne 0$.

Since $x^{-4}=\frac{1}{x^4}$, dividing 1 by it gives $1÷\frac{1}{x^4}=x^4$. A negative exponent sitting in the denominator moves up to the numerator as a positive one; it never puts a minus sign on the answer, so $-x^4$ is the trap.