Eigenvalues · Easy problem

An invertible matrix $A$ satisfies $A\mathbf{v} = 4\mathbf{v}$ for the vector $\mathbf{v} = \begin{pmatrix}1\\1\end{pmatrix}$. Find the number $c$ with $A^{-1}\mathbf{v} = c\,\mathbf{v}$.