Board Thread:Questions and Answers/@comment-179.185.224.243-20171019033702/@comment-32671756-20171019135943

The spawn of 4 Pirate Captains is quite random, I've gone through like 3 Battlemaster Settlements before finding the last one.

If you can only find Commander on that island then you need to try your luck on the other islands, since saving & loading wouldn't respawn the pirates, while sailing to quite far away places and returning will respwan these pirates and loots.