You haven't registered your iPod on four computers, you've registered your iTunes account on four. Even if you haven't owned four computers (PCs or Macs) this will happen if eg. you let someone else view video or listen to audio you've bought from the iTunes store.
You can deauthorize any particular computer at any time from iTunes running on that computer.
If you no longer have access to a computer you've authorized, you can deauthorize all registered computers from your account profile in iTunes. You can do this only once a year though.