I was about to ask how to get this to work, but I just figured it out, so here is an FYI, if you have also been trying to set this up without success.
The Screen Sharing system in Leopard will work with 10.4 systems if they have the software firewall turned off, but what do you need to do if you have it turned on? The Leopard Help is of no use here as it only describes what you need to do for a Leopard to Leopard link so here goes:
1. On your 10.4 system with its software firewall running, turn on the Apple Remote Desktop service in System Preferences>Sharing>Services.
2. Next, click the Access Privileges... button and turn on the accounts you want to be available for control (you will need to have the Apple Remote Desktop service highlighted to see this button).
3. Switch to the Firewall tab and you will see that the ports for ARD are open. Here's the rub to get this working - you will also have to open the VNC ports as well. To do so, click the New... button and choose VNC from the Port Name: pop-up menu. This will populate the port numbers required for you. Click OK. Check that VNC is On.
4. Assuming your G4 is showing up as part of your local network on your 10.5 system via bonjour, initiate the screen share and you should be prompted for the username and password of the account you want to share the screen of. Input that and you should see your 10.4's screen on your 10.5 system.