then amiga with ECS or AGA are not amiga because ECS and AGA were outdated and far behind mac in 1990 and mac,pc in 1992.
Every computer/console that commodore made with an Amiga logo is an Amiga. The Amiga Technologies 1200 & 4000T are also Amiga's because they were pretty much the same as the commodore ones as they were made from left over parts and the old designs.
AmigaOS 4 can run on Amiga's with a PPC board, but it can also run on non Amiga hardware. For example the X1000 is an AmigaOne, not an Amiga.
http://en.wikipedia.org/wiki/AmigaOS_4#Compatible_hardware
