I'm not familiar enough with the 4.0/4.2 oil paths to say one way or another, but bearing failure and piston galling are generally an oil issue in my experience. Sensor pressure is tricky because it just means there is enough restriction in the galley you are plumbed to for the pump to float the regulator. However, the regulator can mask slight loss in a pathway if the loss is still less than the difference in pump volume and regulator setpoint. The flip side is blockage; the regulator and pressure sensor will generally not give you any sign that a galley is blocked if it's on the downstream side of the sensor port. For example, if your sensor is in the middle of your oil galley (assuming all branches parallel) and the 5 and 6 main/rod journal paths are blocked and not receiving nor losing oil, the sensor will still show perfect pressure, if not slightly higher pressure due to overall leakage being less than normal. Not saying this was the case in your instance, but excessive bearing material in oil is either poor oiling or significant trash in the system. Tolerances of bearings during assembly could be culprit too, but I'm going to assume it was built correctly for argument sake. Galling of pistons is generally poor fitment (either because of heat, or causing heat) or lack of lubrication. Teflon helps, but it only reduces friction and doesn't necessarily help the heat cycle expansion tolerance which could be the greater issue if not fitted properly.
Take all of this with a grain of salt because I am NOT an engine builder; just a snotty kid with a Mech Eng degree who thinks he knows everything. The only way to really know what happened is to tear it down and do proper root cause analysis. I just like to speculate in the meantime to see if I still have any brain wrinkles.
