I'm not quite sure whats up with HPI and this parts thing. I'm a new Pro4 owner. Hell I haven't even gotten to run mine because of weather issues, but I also run the R40. I don't know why it takes them a couple of months to catch up, but if it makes any of you feel any better, HPI did eventually catch up with the R40 parts thing. You can now find parts for it just about anywhere or at your local shop.
Hopefully they will catch up with the Pro4 soon too.