You need a 3.5" IDE (Parallel ATA) drive in this machine
IIRC the G4 models prior to the Quicksilver 2002 model have a restriction on the largest capacity of drive the disk controller can handle, 128 GB
Here's a list of G4s and their specifications
Apple Power Mac G4 Specs (All Power Macintosh G4 Technical Specs) @ EveryMac.com
There is no restriction on capacity of a Firewire hard drive, only drives that are directly connected to the logic board by IDE
You'll also need the install disks for the machine with the operating system - first to boot the machine so you can format the hard drive to Mac HFS+, then to install the Mac OS.