I bought Amiga OS 4.0 back in 1987
1987? That's pretty impressive, considering it wasn't even released until 2004. :lol:
As far as answering your question I have no idea if you can install 4.0 and then run whatever updater it includes a million times to update all the way to the latest 4.1 FE. Personally, if it were me, I'd just pay the money to get the latest 4.1 FE disc and install from that. Will probably give you a much more stable and smooth installation process, if nothing else. Good luck!
