All in all, I really enjoyed this tease series. I really like the theme of your desires trapping you in a situation that leads to an ultimate moment of crossing the threshold into permanent denial. I thought you did this quite well, and would welcome any further explorations into this theme from you or any of the other authors here. Very good job indeed.
As far as a part 4 finale, part 3 certainly
seemed like a finale, but I certainly wouldn't stop you from extending it to another chapter. "Ideas", you ask? Well, far be it from me to add ingredients to a soup that tastes just fine as it is, but I'll give it a shot. In the finale, you could assume the intrepid hero was resilient enough to withstand the "final" assault in part 3, clinging on to just enough physical fortitude to someday recover.
Naturally this would be discovered in short order, in which case a closer would be sent in to finish the job. Someone skilled in not only making sure the victim's orgasm is truly their last, but taunting them every step of the way, locking the cold, inevitable reality in their mind as it draws ever closer. If I may, I would also suggest the closer make good on the ruined orgasm threat made in part 3. It could be argued that a final orgasm that isn't truly experienced when desperately needed is more mind-shattering than a fully realized one. When the victim is lying defeated, panting in the aftermath of a ruined orgasm, knowing he will never be physically capable of cumming again - I would say the "Breaking You" mission is complete.
At any rate, just my ideas, feel free to do with that what you will. Thank you for the great tease, with or without a part 4, and I look forward to seeing even more of your work.