Replacing hard drive in G4 Powerbook?

It takes a standard 2.5" IDE drive (as long as it isn't one of those old really thick ones), and as you already seem to know, you'll need to put an OS on it, of course. Yes, you'll need to reformat the drive. Once it boots from the CD, go to the Utilities menu and select Disk Utility. Once it loads, click on the drive on the left, then click the "Partition" tab. Click the "Options" button and make sure it's set to Apple Partition Map. Then set the format to Mac OS Extended (Journaled) and format it. Now you can put the OS on it.