Classic is OS9 running
under OSX; you start OSX first and then OSX runs OS9. Classic is not running the computer the way it does when running
as OS9, OSX is. Your computer is too new and OS9 is not capable of running your computer on its own, but OS9 will work if OSX is really running the computer and helping it.
Classic requires version 9.1 or greater. Many old CDs came as version 9.0.4 and won't work as Classic without an upgrade (free). If you have a CD you have to do two things. One is to get OS9 off the CD and onto the computer. With older G4s this is easy because you can put the CD in the computer, start the computer from the CD and then run the installer. Your computer is too new and won't do that. Even if you could, you would run into problem two which is you still need to run the updaters to get it to version 9.1 or greater.
One way to get around all this is to find a computer that has a copy of OS9.1 or greater on it and get a copy off that.
So the basic answer is, yes you can install, and your computer will run, Classic, but you don't really have the components at hand to easily get it installed on your machine.
(I hope this isn't a troll.)