For running old games, your best off with a mildly expanded A500 or A1200 (WinUAE even).
OS4 is essentially a development of the Amiga OS, intended to run on some select PowerPC hardware (like the "AmigaOne" board or Blizzard/Cyberstorm PPC cards). OS4 is cool if you're really into the OS aspect of classic Amigas, but it won't run classic games. (Not to mention, the hardware needed to run OS4 is somewhat rare and expensive.)
