I'm assuming that the G4 isn't running leopard?
Um, not trying to be insulting, but if you look at the post you quoted while saying this, it says "...my Leopard G4 PowerBook." So obviously yes.
And I've used Safari relatively frequently with both Leopard and Tiger before, and not seen any significant issues with it, nor has my wife ever complained, and she uses Safari exclusively and watches a lot of video, so it's obviously not a "standard" problem.
RawStory's video (and YouTube, and CollegeHumor, and YourFileHost's new embedded player, and every other Flash-based inline video player I've tried) works fine for me in Safari. In fact, since Camino chokes on Flip4Mac's WMV plugin and Safari doesn't, I end up using it as my "video" browser so I don't have to switch every time I run across some embedded WMV, and I have no complaints.
This doesn't help people with the OP's problem, of course, but it sounds from the solution that it's some sort of caching issue, and that is Safari's one weak point--it's really aggressive with caching, so if something weird gets in, it's harder than you'd think to get it out. I see that when testing sites I'm working on all the time. Makes for faster loads when all is well, I assume, but there's certainly a tradeoff.