Permutable Prime with more than 3 Digits is Probably Repunit

Theorem
Let $p$ be a permutable prime with more than $3$ digits.

Then $p$ is repunit.

It follows that the next permutable prime after $991$ is the repunit prime $R_{19}$:
 * $1 \, 111 \, 111 \, 111 \, 111 \, 111 \, 111$