usually if you posses the disc that was used to install it (either Apple-Shipped or Retail) then it means you own it. Just hop onto eBay and find a 10.4 disc. It will most likely have shipped with 10.3 or 10.2, but since the G5 shipped with 10.2, i would think 10.2 was the original OS.

Anyways, Tiger isn't *too* expensive. BTW: what speed is the G4? you *may* be able to get Leopard on it and run the latest and greatest!