Q: Is this possible

Sorry, no, the iPad cannot access the hard drive on your G4.

No, you can't do it with an iPad. Either remove the drive to put it in an enclosure or connect it to another computer using Firewire ports (both machines) or connect both to your local network and use File Sharing.

iOS does not natively support access to your computer's hard drive, but there are third-party apps that do provide such capability if you set up file sharing on your computer. FileBrowser is one that is commonly recommended (I have no personal experience with it myself). 

http://www.stratospherix.com/products/filebrowser/  

Confirm that it can connect to your version of Mac OS X; a very old version may not be supported.

If the iBook won't boot, then you can't access the drive. As Kappy suggested, pull the drive out of the iBook and connect it to another computer to access the data. That will probably be a better option in any case unless all  you want to access are a relatively few documents.

Regards.

Use File Sharing:

Mac Basics- File Sharing

Another alternative is to copy your files to an external USB drive, then restore the copies to your new computer.

Put it in an external enclosure and connect it through USB to router if it supports that.

No. Windows can neither read nor write and OS X disk without using third-party software such as MediaFour's MacDrive.

Yes. You can copy it all to an external USB drive.

That's what we are telling you. Take it out of the computer and put it into an external enclosuse that allows you to plug it into another computer and use it as an exteral drive. You will still need to connect it to a Mac to be able to read it.