when you play khrysalis eventually you will get a little alert from that mantis mirrormask ( the one you talk to when you're just starting out in khrysalis) and she will talk to you about someone strange somewhere in the bastion and asks you to investigate.
upon investigating you find that is a fellow ravenwood wizard sent by Merle Ambrose to help you out and he will give you the forth potion.
After you have freed Dyvim Whiteheart, there's a side quest given by Zaltana called Message in a Bottle. Then you go talk to Allan GhostDust in Bastion.
Did you get the quest to talk to Allan GhostDust? That's how you get it. I believe that quest is given by Zaltanna the Mirrormask. When in doubt, Google it.
wizard 
