Hi Sanjay.
I would recommend better start with jbpm 3. jBPM 4 would be too risky for me at the moment.We started a customer project recently with jbpm 3 but made sure, we can switch later on easily to jBPM 5 or Activiti. See http://www.bpm-guide.de/2010/04/17/abstracting-the-process-engine/ for details...
But it is a bit hard to decide exactly now, best is to wait some months ;-)
Cheers
Bernd