OS4 needs a PPC CPU, so either you buy an A1200 with BlizzardPPC, an A4000 with Cyberstorm PPC, or an AmigaOne system. Or you find yourself one of the 4 PPC developer boards and push that on a Blizzard 2060 accelerator, like the one you can find in the photo album on this site.
To give a short answer: your A2000 will never run OS4.
