Having a benchmark rating does NOT mean that someone is at the top of their level. If someone plays at Districts as a 3.5 and then gets bumped at the end of the year. Their new rating will be 4.0b - even though they have never played a match at 4.0.
Anyone with a 4.0b is either a 3.5 that played in the post season and got bumped or a 4.0 that played in the post season and didn't get bumped. Those are the only conclusions that you can draw.