Right now the ONLY systems OS4 runs on (publicly) are the AmigaONE line of computers. Unfortunately they're no longer being made. However Trokia should be putting out their own systems (Amy'05) soon (hopefully).
In Beta, OS4 runs on any PPC accelerator available for classic Amigas. When OS4 goes final (also hopefully soon), it should be able to use it on any PPC Amiga. As far as I know, there are no "minimum" requirements for OS4 in terms of speed. I'm sure OS4 will be quite speedy even on the lowest spec'ed PPC accelerator. However, obviously, the faster the better.
As someone else has already stated, there is work on a new PPC accelerator for the A1200 called the PowerVixxen (I'm praying for an A4000 version to put into my system). That should run OS4 as well but when this card will come out is unknown.
Unofficially, there are hints that OS4 is running on other systems. I hope this is true but who knows. :-?
