Where is the PMU button the the Power Mac g4?

PMU stands for Power Management Unit - http://en.wikipedia.org/wiki/Power_Management_Unit.

If you look at the bottom of the tech article you referenced, there is a section on where to find the button for several Mac models including G4s. If you don't see it, this is the link for a g4:

For Power Mac G4

http://support.apple.com/kb/HT1939#where