Q: PowerbookG4 has OS X 10.4.11. What is latest OS X that can be installed? Can I install OS X 10.6 without installing OS X 10.5 since that does not seem to be available?

The latest you can install is OS X Leopard 10.5.8.  Snow Leopard 10.6 requires an Intel processor.

http://www.hardcoremac.com has unopened Apple software. And the used market is possible, just stick to the black-faced retail disks, as the gray-faced disks are locked to the system type they are delivered with, and Leopard was never delivered with any Powerbook.

Mac OS X 10.5 Leopard installation system requirements

http://support.apple.com/kb/TA24950

Leopard is no longer available at the Apple Store but may be available by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753).

If you can't obtain a retail install DVD from Apple, look on eBay or Google the installer part numbers to possibly find at an on-line store. Here's what to look for:

MB427Z/A  Leopard 10.5.1 install DVD

MB576Z/A  Leopard 10.5.4 install DVD

MB021Z/A  Leopard 10.5.6 install DVD (single user)

MB022Z/A  Leopard 10.5.6 install DVD (5-user family pack)

Installing Mac OS X 10.5 Leopard

http://support.apple.com/kb/HT1544

Mac OS X 10.5 Leopard Installation and Setup Guide

http://manuals.info.apple.com/en/leopard_install-setup.pdf

After you install the base 10.5, download & install the 10.5.8 combo update at http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update

 Cheers, Tom