I visit all 4 at various times. The least visited is GreenHybrid...just too vague IMO, trying to cover too much ground with no real focus, little activity. Great format, nice layout, lots of good things, but I wish Jason would rework the site with a lot more focus on the Prius.
I go to the 2004-YG frequently and actually tend to keep up with it (I know, get a life, I'm trying). There are a few resources there that make it remain valuable and the sheer volume brings some interesting and informative topics if you can manage to separate the wheat from the chafe. Wayne Brown is awesome, I wish he'd come here or the PriusChat site.
PriusChat is the new big kid on the block. Danny works hard to make it a great site. There's a great group there, little bickering, people stay on topic yet have fun with outside things in the "off topic" section. It's easy to navigate, easy to find threads you want to follow and ignore the ones you don't. It remembers what you've read and haven't read from one visit to the next. Lots of bandwidth, for the time being, for pictures and articles and such. My current favorite.
Finally PriusOnline...I can't, for the life of me, understand why it isn't more popular than it is. Good knowledgeable group, decent format though there are a couple quirks making it a little less friendly to use than PriusChat. Good Prius focus while making room for other topics and cars without distracting.
I'll continue to concentrate my efforts, contributions and time primarily to PriusChat, secondarily to 2004-YG, and keep an eye on this place to see what's happening. Green Hybrid deserves and occasional check-in, but I'm yet to be convinced it has legs.
FWIW, I've looked in on the Edmunds forum (too hard to follow, but some smart folks there), CarTalk--little of interest, and a couple others, but they're all more or less there because every other car in the world is represented and not b/c there's a dedicated following.