The problem is that OS4 is hard coded to only run on AmigaOne boards (which you can't buy) and Classic Amigas with PPC boards (which you can buy, but the OS hasn't been released). The only way to get it to work on different hardware, like a Mac, would be to either hack the OS to remove the restriction (which would be illegal) or to emulate the missing hardware so OS4 would think it was running on an AmigaOne (which can't be done ATM).
Also, even if you could get the OS to boot on a readily-available piece of hardware, you wouldn't be able to distribute it legally. So basically, we're stuffed until AInc get their act together and release something.
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moto