For $a_n = 4a_{n-1} - 4a_{n-2}$ with $a_0 = 1$ and $a_1 = 4$, the characteristic equation is $(r-2)^2 = 0$ (repeated root). Using the closed form $a_n = (c_1 + c_2 n) \cdot 2^n$ and the initial conditions, find the formula for $a_n$.