G4 processor

Ziploc wrote:

I want to upgrade the OS on an older Mac Mini with a G4 processor. What is the highest version of Mac OS I can use?

The highest i.e. newest version you can run on your G4 Mac mini is Mac OS X 10.5.8

I am using a Power Mac G4 with 350 MHz processor

OS 10.3.9

What's the highest OS I can upgade 2

You can upgrade to Tiger 10.4.