PC-100 and PC-133 are both the same RAM type, but run at different speeds depending on your system bus. Your G4/500 has a 100 MHz system bus, so you'd need PC-100 SDRAM - however, you could use PC-133 SDRAM, but it'd run at PC-100 standards. PC-133 SDRAM is designed for 133 MHz system bus Macs.
Make sense?
Your G4 can hold 2 GB of RAM, but the MacOS can only use 1.5 GB of RAM, so don't add 4 x 512 MB DIMMS.

Just some useful advice.