OS 3.9 is the last release which runs on classic Amiga hardware. 4.0 and above need PowerPC processors. 4.0 is available for PowerPC accelerators, 4.1 and on need complete new computers (AmigaOne, Pegasos2, SAM).
Note that 3.9 needs some expansions to run: 68020 processor, 8 MB RAM, hard drive and CD-ROM. It does not run on an unexpanded A1200 for example.
