Just google CP4 failure and you can read for days. So, the CP3 that Cummins has run since common rail came out in '03 is very reliable, and if it does fail it just quits and you change it out. Ford and GM have been running the CP4, and it does not do well with the ULSD fuel in the US, it is much, much, much more sensitive to fuel quality issues, and due to the basic design, when a CP4 fails it makes shrapnel that ruins the lines, the rails, the injectors, the relief valves, the rail pressure sensors, etc. So basically it ruins the entire fuel system. GM finally got away from it and went to a Denso system. Ford offers a kit to convert to a CP3. What does that tell you??? Now there are rumors of Ford getting away from the CP4, too.
There is talk that it has been internally improved, but I have seen nothong to confirm it. The fact that it says CP4.2 does not mean it is a unit revision. Plus, and most importantly, I have seen nothing that leads me to think it is even possible to prevent it from ruining the rest of the fuel system in the event of a failure.
I wish Ram and/or Cummins would address this directly. They are not fooling anybody. People who have been around these motors are well aware of the CP4's terrible reputation in the industry. They are risking a HUGE black eye in the name of a cheaper component.