\[\frac{4 \cdot 18^{n}}{3^{2n - 1} \cdot 2^{n + 1}} = \frac{2^{2} \cdot 9^{n} \cdot 2^{n}}{\frac{9^{n}}{3} \cdot 2^{n} \cdot 2} = \frac{2}{\frac{1}{3}} = 6.\]