The fourth attack shown in the details section of this page represents the charged attack, however the charged attack can be initiated at any point in the combo.
When striking the back of a Thorny, the player is not always injured. The charged attacked enables the player to bypass the protection provided by their back and deal damage without injury.
The damage from the charge attack comes directly from where you stop moving, meaning that if a Shieldbearer is facing you when you start the charge, it will damage the Shieldbearer once you finish. This works the same way for every other enemy that can block attacks.