You didn't say which g4 you had, but you may have an issue with that large of a hard drive. At least I should say you might not be able to see the full 200GB of storage. Older G4 systems only recognize 128GB no matter what operating system you are using. I think it was the last of the G4 Quicksilver line (867, 933 and dual 1.0ghz) that broke the 128gb barrier.
Aside from that, OS X.4 will be waaay faster than any previous OS. Unlike MS Windbloze - each update Apple has issued has made most machines faster from G3's on up. 10.2 was faster than 10.1, 10.3 was faster than 10.2...etc.