Exactly, killing four exposed men in rapid fashion without killing the hostage is doable.
Yup, the experienced sailors involved (both hostage and rescuers) just understand that everyone waits for the captain to grab an opportunity to jump overboard for a few seconds to give a clear shot, at which point the remaining pirates are immediately taken out. They’re all smart enough to figure that out, we just have to wait for it to happen.
...oh wait, the captain did figure it out already, lept overboard already, gave the clear window of opportunity already, and nobody else did squat most likely because of orders from on high, leading to the captain being re-captured and presumably more firmly restrained. Opportunity taken, squandered, and gone.