Peanut, give it time for additional phases. It would be ludicrous for them to spend $$$ on research, tooling, molds, marketing, etc for just Ph II and Ph III.
I believe these cars are going to pave the way for new Amtrak models (Horizon, Viewliner, etc.) <--- personal opinion.
But as for other phases, I am willing to bet vital parts of my anatomy that Ph IVb will debut on these models. (Look how quick the AMD-103's sell out!)
But as I have said before, PLEASE keep the requests going to Walthers and let them hear your desires for other Phases, it only adds fuel to the fire.
Please thank them for their "Amfleet project", we have waited a LONG time for this, and many (including myself) wondered if it would ever be undertaken. I cannot thank Walthers enough for this.
Tom