I'm aware of how they were related... AS3 was based on ES4 ideas, and, as the major implementation of ES4, AS3 influenced the evolving direction of ES4. But ES4 was never really finished to the point where everyone who needed to actually agreed on it.
I think MS, who had a browser monopoly at the time, was never going to agree to something that made Adobe Flash more important. It seems crazy now, but at the time it seemed like a real possibility that browsers could end up mere shells for the real internet runtime, Flash Player.