Sounds like if you can win a 4.0 tournament you have no business playing 3.0.
I'm kinda in a weird situation myself. I started playing tennis again in August after 2 years off. I was playing 3.0 and 3.5 players in singles ladders and was getting my butt whipped by 3.5 and winning most 3.0 matches but losing a few really bad. I've been playing and practicing alot since and am now playing well against 3.5 level players in singles. Although I joined a 6.0 winter mix as a self-rated 3.0 that starts in january. This weekend I played at this tennis social where you play doubles and they rotate people so you end up playing with a variety of people. I was playing with alot of 3.5 players (one was 4.0) and was playing very well, even dominating people with my serve. The last group I played with, I talked to one of the opponents for a few minutes and he was shocked to hear that I was registered to play at the 3.0 level (6.0 mix). He said he is a 3.5 and actually considered me a 4.0 by his standards. He said I was cheating and I explained that I actually wanted to play 3.5, but couldn't find a team that was interested in me because at the time I was looking for teams, I advertised that I was a strong 3.0 - weak 3.5. I've improved since then to where I feel guilty playing at the 3.0 level because my serve and forehand are way above 3.0 and might **** some of my opponents off. Since I've never played USTA, I don't have a computer rating so I can only go by what my opponents tell me about their own ratings. The good news is that I cannot be disqualified from the mix league, but in the Spring I might join a 3.5 league even though my rating will still be 3.0. It becomes apparent to me more and more as I play different people that I am probably sandbagging, but I guess I can't be too sure until I start playing official matches.