Great read, thanks for sharing the story.

I think I echo the others here, four rounds isn't bad at all. Not exactly the same thing as a YC application, but raising our seed round @ Dasher we were rejected 48 times from angels and VCs before we got our first "yes." Startup myth goes that Pandora's founder was turned down 300+ times.

I'm also very impressed with the specific attention YC paid you when turning you down. Rather than a generic answer, they took the time to give you very thoughtful reasons and things to improve. I really wish we had that much care paid to us by anybody during our earliest conceptual days.