4. The flea Kuzya can make jumps from each vertex of a regular tetrahedron $A B C D$ to the three adjacent vertices, and the choice of these vertices is random and equally probable. Kuzya started jumping from vertex $A$ and, after 2019 jumps, ended up at vertex $B$. With what probability could this have happened?
Answer: $\quad P(A)=\frac{3^{201}+1}{4 \cdot 3^{201}}$.