I know you can install Ubuntu on it, but as for FC4 I'm not sure. They don't have any wikis on how to install on OldWorld Macs (like your PowerBook, for instance).
OldWorld and NewWorld refer to Macs that use a hardware ROM (OldWorld) or OpenFirmware (NewWorld). OpenFirmware is similar to a PC's BIOS, but is much better. OpenFirmware simulates the Mac ROM in memory, so it's all software based as opposed to a piece of hardware ROM.
Macs that are OldWorld can't boot other operating systems from a CD other than Mac OS. If you were to install Linux, you would have to use a bootloader that runs on the Mac OS like BootX. Not a big deal since you wouldn't need a large partition for the Mac OS...just enough to get it installed and have BootX and the kernels there. There is also an extension that is included with BootX that kicks in BootX before any other extension or control panel is loaded, allowing you to boot right into Linux once the Mac OS boot process has started. There's also another bootloader called quik that will allow you to have a Linux only system, but it's rather risky and could leave you with a doorstop.
NewWorld Macs are those that have AT LEAST USB capabilities BUILT-IN to the logic board. They will boot from a Linux CD (for the most part) and don't require a Mac OS to be installed. This means you can have a Macintosh computer with Linux only if you wanted to. Yaboot is the bootloader used in NewWorld Macs and resides in OpenFirmware.
Now, it might be possible to install FC4 on your OldWorld PowerBook, but it's goign to take some work. I have Debian installed on my StarMax and I've thought about installing FC4 on it, but it's such an old machine that to go through the trouble isn't worth my time. Of course, that might not be the case for you. Give it a try and let us know what you need help with.