To sum it up, if you want it to add to your collection and use it with its limitations then go for it. I have spent a ton of money on relics while getting newer machines for free (funny what people will throw out). If you just want to do programming, office tasks, light video encoding, music , etc then a G4 is fine for that. Anything it could run when new it can still run now (and much better if you have newer HDs and maxed out RAM).

If you look at it historically when the internet became multimedia enabled all the old machines designed and built before that time tended to be obsolete for that use. Any machine that might hold up to the challenge but was abondonned bu the manufacturer would also not be a good platform for major web use. The G4/G5 has been obsoleted by Apple, so being a closed platform with no major support its not the best choice for web/video/movies. Even old P4/Athlon 64s of the same era have decent support for video even today. It used to be Apple supported machines way past their prime (OS 8.1 running on a 68K for example way after PPC had taken over), that is not the case anymore.

The good thing about older macs pre x86 is people are tossing all the old apps and hardware so you can be a retro geek on the cheap with a full complement of legit software for most needs.