If you performed an 'Erase and Install' installation of MacOS X 10.4, Tiger, onto your PowerMac G4, System 9.2.x was not installed as part of the process.
You must install System 9.2.x yourself. To say 'Classic' is to say - 'System 9.2.x - limited in functionality'.
If your Mac can boot from System 9.2.x, and you did such - you will again experience - the once fast, quite productive, and most user friendly operating system, of all times.
When you run Classic from within any version of MacOS X - you are running a crippled System 9.2.x. Classic will still be faster, quite productive, and very user friendly - from a MacOS X perspective; however, a number of applications will not run due to System 9.2.x's crippled state.