can I install classic os on powerbook g4?

If you have an aluminum model, only if it’s running Mac OS X 10.4.11 or earlier.

If you have a titanium model, you can either install Classic as above or boot it to Mac OS 9.

In either case, you’ll need to have a DVD which contains the Mac OS 9 files.

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Niel is correct. For both options, see this tip:

Mac models capable of dual-booting into 10.5 and Mac OS 9