OS4 pre-A1/Teron, is not the same OS 4 as now. (and you know that)
What would of been huge chunks of the OS4 upgrade were already well-progressed at that time*, and predated even the "on-schedule and rocking" period. Three months was not inconceivable for OS3.9PPC + BB4 a-la H+P's original plan.
For whatever reason the plan was revised while waiting for the hardware, perhaps it was the threat of MorphOS, perhaps it was to satisfy a potential customer of the time. No-one except those making the plans knows.
In some ways I'm disappointed with this choice, in others I think it was the right thing to do.
*independently of Amiga Inc., whether as a continuation of 3.9 or separately I don't know.