You went undefeated at 3.5, so you shouldn't be there anymore. You'll lose more than you win as a low 4.0, just like almost every other low 4.0. The system is working as designed. I don't really understand how this creates "purgatory."
I don't fault the system for moving me up; I know my results dictate that. It's just the fact that I'm a bad doubles player combined with the fact that I will be playing much better players that makes me feel that I'm not quite ready for 4.0. I don't see it as a flaw in the system, but think I'm too good for 3.5 singles and not good enough for 4.0 (singles or doubles) at the moment.