Keep in mind that while in classic you are running a simulator. And that simulator uses a heck of alot of memory.
My suggestion is to quit Classic when you know you are going to walk away from your computer for any long length of time.
The hard drive would not be taxed. Energy Saver is not designed to save the hard drive but to save your electricity consumption when you are away from your computer.
One more thing, exactly what model G4 do you have? There is an update for the iMac's to correct a problem exactly like yours.
Check the Apple updates page.