A mapping `f : \mathbb{R}-{1/3} \rarr \mathbb{R}-{1/3}` is defined by `f(x) = \frac{x+3}{3x-1}`. Then the value of `f^{-1}(4)` is -- (a) `{11/7}`, (b) `{1/4}`, (c) `{7/11}`, (d) none of these.