Is the disk a full retail install disk like this one:
If it is the gray system install/restore disk from another Mac, it may not work. If the disk is one shipped with an Intel Mac, it won't work at all. Also, there are some disks that look like the picture but are marked "up-to-date." They were shipped with Macs that were bought just as a new OS was introduced. They are also model-specific and will only work for the machine with which they were shipped.
I'm going to ask some other upper-level helpers to see if the type of install you are trying is compatible with an eMac. The articles refer only to Intel-based Mac Minis and Macbook Air computers that do not have any optical drive.
EDIT: OK-- I put our a "Bat Signal" to other experienced posters asking they comment/advise.
First off, backup any data off your machine to a external drive and disconnect all drives before continuing. Standard warning.
Next perhaps use Cabon Copy Cloner to clone your entire 10.4 drive to a another blank external drive, the drive has to be formatted the same as the 10.4
Check with CCC/Disk Utility as I forget what the old 10.4 formatting was, Apple Partition Map or something.
But CCC might just keep the same old format when cloning. (CCC is 10 years old, so it was cloning OS X at that time)
You also perhaps need to know how to boot off this external clone of 10.4, it will contain everything like before so you can experiment further, likely holding
likley something in open firmware as PPC machines have no EFI.
Perhaps follow these directions to create a 10.5 boot USB, I haven't tried it myself.
10.5: Install on a PPC Mac from an external USB Drive
If you get it to work you can also decide to update the external clone, boot off of that to check it out, even erase and reverse clone or whatever you decide as your working with a copy. Clone that to the internal.
Perhaps buy a old eMac for parts and replace the optical drive.
Some ideas anyway. Good Luck, hope this helps.