Answer:
[tex]\large\boxed{f^{-1}(x)=\dfrac{1}{4}x}[/tex]
Step-by-step explanation:
[tex]f(x)=4x\to y=4x\\\\\text{exchange x to y, and vice versa:}\\\\x=4y\qquad\text{divide both sides by 4}\\\\\dfrac{x}{4}=\dfrac{4y}{4}\\\\\dfrac{1}{4}x=y\to y=\dfrac{1}{4}x[/tex]