What external hard drive works with my 10.4.11 G4

All that are USB 2 or 1.1 or Firewire will work to some degree.

USB will not boot Mac OS X.

USB ports on the rear of the machine as opposed to the side will only work at 1.1 speeds (15 Mbps).

If you format the drive GUID or ExFAT it may not even be usable. Use HFS+ Journalled if you want it to boot.

Use FAT32 if you want to transfer files between PC and Mac without networking, but don't mind the lack of booting.

You can't easily make a drive that can both boot Intel (2006 and later) and PowerPC Macs.

Manufacturer backup software included with many drives won't work.

Use Apple's Disk Utility on said Mac to format it according to needs. This will wipe its contents.

Only http://www.macsales.com/ (that I'm aware of) provides software with drives that is worthwhile keeping, if you request it come with their old PowerPC software suite.