For any integer $n \geq 0$, the given implies $x^{n+3}=-4 x^{n+1}+8 x^{n}$, so we can rewrite any such power of $x$ in terms of lower powers. Carrying out this process iteratively gives $$\begin{aligned} x^{7} & =-4 x^{5}+8 x^{4} \\ & =8 x^{4}+16 x^{3}-32 x^{2} \\ & =16 x^{3}-64 x^{2}+64 x \\ & =-64 x^{2}+128 . \end{aligned}$$ Thus, our answer is 128.