Not sure to be honest Nathan – i’ve always assumed that the likes of Arazi and Jo’burg were stronger, faster horses at 3 than 2, but had only improved a small amount compared to their peers so had been caught up and surpassed by less precocious juveniles.
I have always been under the impression that filles are more unlikely to train on from 3 to 4 than a colt, but that could be complete nonsense.
I have also read on here people doubting, for examply, Frankel "training on" from 3 to 4. Given that over a mile he would race horses a year older than him at levels rather than recieving 3lb (as he did as a 3yo in the QE2), i assume that the Powers that Be expect 4yo’s to perform 3lb better than they did at 3yo.
I must also assume that as there is no WFA for 4yo’s than the Powers that Be believe a horse (male or female) is at its physical peak at 4yo. I have been told by others that it is 5, hence the "name change" previously referred to.
I’d need someone a bit more informed to confirm either way to be honest.