Q: PowerMac G4 OS?

No where free that's legal or maybe trustworthy.

I doubt Apple still carries thwm but you can call Apple with the Serial# handy & see..

How to find the serial number of your Apple hardware product...

http://support.apple.com/kb/HT1349

10.3 isn't really worth much these days.

Tiger Requirements...

To use Mac OS X 10.4 Tiger, your Macintosh needs:

    * A PowerPC G3, G4, or G5 processor

    * Built-in FireWire

    * At least 256 MB of RAM (I recommend 1GB minimum)

    * DVD drive (DVD-ROM), Combo (CD-RW/DVD-ROM) or SuperDrive (DVD-R) for installation

    * At least 3 GB of free disk space; 4 GB if you install the XCode 2 Developer Tools  (I recommend 20GB minimum)

http://support.apple.com/kb/HT1514

http://www.ebay.com/sch/i.html?_nkw=mac+os+x+tiger+retail+10.4

See Tom's, (Texas Mac Man), great info on where/how to find/get Tiger...

https://discussions.apple.com/message/15305521#15305521

old: http://discussions.apple.com/thread.jspa?messageID=9755670&#9755670

Or Ali Brown's great info on where/how to find/get Tiger...

http://discussions.apple.com/thread.jspa?messageID=10381710#10381710

Or join my site & ask...

http://x704.net/bbs/