DeltaMac has a great suggestion for you!
Another way, but I don't know if you're able to do this, it's a bit advanced:
There are many files in the Leopard installer that you don't need on there. i.e. Printer drivers, etc. if you can cut down everything you don't need, it should fit on a 4GB disc. You have to know how to navigate this stuff, and which things to DEFINITELY NOT remove. right/control-clicking on a 'package' file will allow you to "show package contents", you'll also need to enable the viewing of invisible items in the finder, I would suggest downloading a program called Tinkertool to do this. Goodluck.
Another way, but I don't know if you're able to do this, it's a bit advanced:
There are many files in the Leopard installer that you don't need on there. i.e. Printer drivers, etc. if you can cut down everything you don't need, it should fit on a 4GB disc. You have to know how to navigate this stuff, and which things to DEFINITELY NOT remove. right/control-clicking on a 'package' file will allow you to "show package contents", you'll also need to enable the viewing of invisible items in the finder, I would suggest downloading a program called Tinkertool to do this. Goodluck.