Q: Install 10.5.4 on PB w/ 10.4.11?

No. You need to use a retail DVD to upgrade.

(65090)

Apple, in the US at least, no longer sells OS X 10.5 Leopard. One source for unopened Apple software is http://www.hardcoremac.com . Otherwise, you need to look at the used market, and as you've found above, the gray faced disks delivered with systems are locked to those system types. Since Leopard was not delivered with a Powerbook, there won't be any gray-faced OS X disks available.

Mac OS X 10.5 Leopard installation system requirements

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

Leopard is the max OS for a PowerPC processor  Mac like the G4.

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

Caution - Leopard does not support classic mode. So, if you currently open OS 9 apps in classic mode, you won't be able to do this if you upgrade to Leopard.

 Cheers, Tom