I think four rounds is too many. I don’t know why people are going on about an uneven playing field; nobody in the 100m who can’t run the B qualifier stands a chance in hell of making the final.
The B time of 10.28 (Beijing) is not fast enough to make the final in any WC or Olympics, and almost certainly not the semi-final either unless one has been injured a lot and is actually quite a lot faster.
In Berlin, the slowest time to make the semi-final was 10.24.
The slowest time that advanced to the final was 10.04.
The playing field is level, as all the guys who stand even a remote chance of making the final will already have the qualifying time.
All the guys who would only ever run one round (old or new system) will still only run one round.
The guys who would only ever run two rounds (old or new system) will still only run two rounds.
There might be one or two athletes who make the semi-final (out of 24) who do not have the B qualifying standard, and they will run only two rounds instead of three in the old system. Any athlete who barely makes the semi-final and who doesn’t have the B standard will never make the final, so the fact that they’ve run one extra round won’t matter.
All the guys who are in the semi-final and stand a chance of making the final will have all run the same number of rounds.
The finalists will all run three rounds.