Amazing what happens when you actually try to use the tool.
Hmm let's add an account. OH MY what account type? Let's see what the drop has to show me...
Mac.com.. hmmm no I don't want to pay for that.....
AIM.. hmm that's free
Jabber.. free again
Google Talk.. again free.
Looking at the tool kinda answers the question. However of the 3 free options, only 1 really takes advantage of the video chat, and I'm pretty sure unless it's Macs/iChat your talking too, it won't do the 4 way conferencing. If one of them is a windows user they won't be able to join the 4 way.
I know my response is kinda harsh but come on, the tool itself answers the question.