System Requirements for OS X 10.4-10.6
OS X 10.6 Snow Leopard
Mac computer with an
Intel processor (You do not have a intel processor so you can NOT run snow leopard on your G4)
1GB of memory
5GB of available disk space
DVD drive for installation
OS X 10.5 Leopard
A Mac computer with an Intel, PowerPC G5, or
Power PC G4 (867 MHz or faster) processor
512 MB memory or more
A DVD drive for installation
9 GB of available disk space or more
OS X 10.4 Tiger
A PowerPC G3,
G4, or G5 processor
Built-in FireWire
At least 256 MB of RAM
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.
Tiger will work for sure

From the photo you posted of the kp it says "PowerMac 3,1" That is a PowerMac G4 Sawtooth so your CPU (If not upgraded) is a 350-500MHz G4 To install Leopard you need to do what is in this link
http://lowendmac.com/osx/leopard/openfirmware.html You will need 512MB Ram or more. You have one of the best G4's ever made The sawtooth will last for ever

. You can find retail OS X 10.4 And 10.5 Discs on Ebay
View attachment 326904
This is the Retail OS X Leopard Disc this is the one you need for your G4