So at this point it's pretty evident that The Hangover Part IV is not happening however there are many requests for it, so one never truely knows. In any case I thought of an interesting idea for a fourth part.
First of all it takes place during and after the events of Part 3. After Sid Garner dies Tracy begins to starts worrying everyone as she begins to show signs of mild depression. To try and combat this Lauren and Stephanie decide to take Tracy on a suprise trip to Vegas seeing as how happy it made Alan. They leave soon after Alan's intervention however it doesn't seem to cheer her up at first and something goes wrong, leading them to get drugged. They then wake up in similar fashion to their husbands to find that Stephanie is gone. In addition to this they also wake up with crazy things happening to them (per usual in the seires) and find Jade and Cassie are there with them. The whole time Marshall and Chow are after Cassie to get the cash for their gold before the law catches them, however Tacy and Lauren need her because she's the key to finding Stephanie. To make things even crazier we find out that Chow's wife is actually Melissa from the first Hangoer who acted as Stu's girlfriend gathering information on Stu for some unspecified reason.