Determine whether the sequences are increasing, decreasing, or not monotonic. If increasing, enter I as your answer. If decreasing, enter D as your answer. If not monotonic, enter N as your answer.

1. $a_n = \frac{\cos n}{2^n}$
2. $a_n = \frac{\sqrt{n + 2}}{8 n + 2}$
3. $a_n = \frac{1}{2 n + 8}$
4. $a_n = \frac{n - 2}{n + 2}$