If you NEED to install OSX 10.5 on an unsupported G4 for some reason, know that there are a couple ways you can force the Leopard installer to boot on unsupported hardware.  Here is the Open Firmware way, allowing you to use a completely unmodified (and even a genuine) OSX 10.5 retail install DVD (this can also be done using an USB stick).  Before we begin, be warned that the reason Apple blocked pre-867mhz G4's to boot OSX 10.5 is because this is going to be slow and painful to use.  Pre-867mhz should stick to OSX 10.3 instead.

1. Got a retail (NOT GRAY) Leopard install (NOT UPGRADE) DVD? Great. Otherwise, get the appropriate disk image here: Mac OS X Leopard v10.5.4 Install (DVD-DL)

Optional: If you want to use an USB stick instead of burning a DVD, first follow these instructions and do NOT click the link at the bottom for instructions on how to boot from Open Firmware.  Instead, just come back to this page and continue with the following procedure:

Optional: (If you're doing this with an USB stick instead of a DVD, insert your USB stick in a root USB port, not in the keyboard nor an USB hub)

2. Turn on the G4 while holding Command, Option, O, and F keys. After 5-10 seconds, you'll see a white screen with black text.

Optional: (If you're doing this with a DVD disk, insert it if it's not already in the drive.)

3. In the Open Firmware prompt, type the following. Press Return after each line. It will say "ok" if the text you entered was correct.

dev /cpus/PowerPC,G4@0
d# 867000000 encode-int " clock-frequency" property
d# 867000000 encode-int " min-clock-frequency" property
d# 867000000 encode-int " max-clock-frequency" property

Optional: If you have a dual-CPU Mac, also type

dev /cpus/PowerPC,G4@1
d# 867000000 encode-int " clock-frequency" property
d# 867000000 encode-int " min-clock-frequency" property
d# 867000000 encode-int " max-clock-frequency" property

4. After you type these lines, type "multi-boot" (no quotes) and select the leopard install DVD.

It should boot up and successfully load the installer. After it loads, click "Utilities" (in the menu bar) then "Disk Utility". You will need to format your hard drive because you cannot upgrade from OSX 10.4 Tiger. Click on your hard drive, then click the "Partition" tab. Click on the dropdown menu and select "One Partition" Name it whatever you want and click the "Options" button. Select "Apple Partition Map". Click apply. Now you just proceed with installation normally.

Source: https://www.youtube.com/watch?v=P77h5HwNwvg