Last I heard, the extension RJ gates will open mid-January 2015. When I passed through JFK last week, the jet bridges were already attached and labelled, but the interior still looked messy and under construction (from what I could see from the window of a plane taxiing by!)
There will be 7 jet bridges on one side (the odd-numbered gates side, hence going up to Gate B55), and 4 on the even-numbered side.
There will also be bus station at the RJ gates end of T4 (just like B18 today for the Jitney to T2); I assume they will run a jitney as well from there to T2.