The recurrence $a_n = 4a_{n-1} - 4a_{n-2}$ has characteristic equation $(r-2)^2 = 0$ with repeated root $r = 2$. The general solution is $a_n = (C_1 + C_2 n) \cdot 2^n$. If $a_0 = 1$ and $a_1 = 4$, what is the closed-form formula?