yep definitely 2 pistons. So who ever told you that it has 4 either doesnt know what they are talking about or they are trying to rip you off.
We have the zzr250 manual in the general tech help section, (its the GPX250 and further down a zzr supplimental) the picture came straight out of that, so i dare a mechanic to argue with that. Also i just finished pulling mine apart and i only saw 2 pistons.
$217 is a bit high if you ask me. It wouldnt take more than an hour to pull that thing apart and together again. I was going to say ill do it for you for free, then i saw where you live... Unless you can get it down to me in Sydney, i cant help you
Like Marty said, if you are getting grief from this mechanic, go to another one. Im sure some fellow QLDers can suggest places to go.