If you went undefeated at 3.5 as a self-rated player I would have thought some 4.0 team would have already taken a chance on you. I think you're selling yourself short in your ability to improve too. I didn't start playing league tennis until I was 47 and after 5 years of being a stagnant 3.5 with a marginal record I've worked my *** off to improve my fitness and technique and got bumped in my local league to 4.0 and I have to believe I'm very close to getting the USTA 4.0 bump as well. I'd was undefeated in 3.5 singles at 53 last year against computer-rated 3.5s but I also kept running into undefeated self-rated 3.5 players such as yourself.
At least I was glad to see that all four of them I lost to was Early Start Rated at 4.0.