*Will OS4.0 be a seperate thing?
The powers that be don't want the hardware and OS sold seperatly due to piracy concerns. Nobody knows how upgrades are going to happen, either.
Obviously that doesn't apply to the classic amiga version. It's not currently available for purchase, but likely it will be the only legal way to buy "only OS4"
But why would you want to, if you don't have hardware to run it on?
