It would help to know which iBook G4 model you have.
Go to the Apple menu, then About this Mac, and click the More Info button.
Your System Profiler will launch.
Under Hardware Overview, what is listed for Model Identifier? That will be something like PowerBook6,5
The largest factory hard drive would have been 100GB, but most iBooks would have 60GB or smaller.
If you want to try the full task, on a near-15-year-old laptop, then I would suggest that you backup the hard drive to an external firewire drive, then boot to a compatible system on an external drive - or boot to an OS X installer DVD. Run Disk Utility, and erase the hard drive (including re-partioning the drive, making sure there is only a single partition.) Then reinstall OS X. You do that from the installer DVD, as that would be its main purpose
Restore your files and apps from your backup disk. That "nuke & pave" of the hard drive CAN sometimes help make the performance closer to what you might like. But, it's still old hardware, running an old system, with internet support and browser support just not as good as you might like. The end result is a system that will always be slower than you might hope for. DON'T compare this to a new system. You will be disappointed.
If that hard drive rebuild takes a long time (more than 3 hours, for example), or fails with an error of some kind, that may mean that the hard drive is failing. It takes a laptop IDE drive (not the modern SATA connection), and the old IDE drives are not easy to find. There are PATA SSDs, although you would need to search for that. You won't ever need to worry about a noisy drive motor or read head failure, as there are no moving parts in an SSD, and also will give a more responsive system.
You will be limited to Leopard (OS X 10.5.8), nothing newer can work on the G4 processor, and some users say that the older 10.4 (Tiger) is a better system on a G4.
BTW, replacing a hard drive is quite challenging. There's 50 tiny screws to remove to get to it (I counted them once when I was working in a shop, as the customer thought it was a simple, 5-minute job), and the laptop is completely disassembled - not just a small door or panel, or something as easy as removing only the bottom case. Don't EVER do this without some method of keeping the loose screws organized, and under control.