\[c_{1} = 4;\ \ c_{n + 1} = 2c_{n}\]
\[c_{2} = 2 \cdot 4 = 8;\]
\[c_{3} = 2 \cdot 8 = 16;\]
\[c_{4} = 2 \cdot 16 = 32;\]
\[c_{5} = 2 \cdot 32 = 64.\]