7. In a regular tetrahedron $ABCD$ with edge length $1 \mathrm{~m}$, a small insect starts crawling from vertex $A$ according to the following rules: at each vertex, it chooses one of the three edges connected to that vertex with equal probability, and crawls all the way to the end of that edge. Let the probability that the insect returns to point $A$ after crawling $n \mathrm{~m}$ be $P_{n}$. Then $P_{4}=$ $\qquad$
7. $\frac{7}{27}$