Minimum hardware requirement is an A1200 with a PPC processor. But OS4 may not run on a BlizzardPPC turbo card because of hardware flaws.
First version of OS4 will run on CyberstormPPC turbo cards running on Amiga3000(t) and Amiga4000(t).
But full performance will be reached on the new AmigaOne`s (and maybe Pegasus)