Since the measured pressure at the cooler inlet is constant, when all this happens, it proves three things (at least in my mind).
1) Since the pressure is not changing, the PRV has opened and it is regulating the pressure.
2) The problem is after the cooler.
3) There is no problem before the cooler. The
oil pump, pan pickup, baffling is not causing the problem or the pressure before the cooler would drop at the same time it does at the engine.
Since the PRV is open some of the
oil (that the pump is pumping) is dumping back to the pan and some of the
oil is flowing to the cooler, and thus into the engine. The point at which the PRV lifts whatever amount of oil is flowing through the engine, will not increase no matter how much the rpm increases. So the oil flow through the bearings is not increasing with rpm (after the PRV lifts).
Oil is heated in the bearing from the shear action created by the speed differential between inner and outer surfaces. The longer the time the oil spends in the bearing, the hotter it gets. The higher the rpm the quicker the oil heats. If the oil flow through the bearing is not adequate, it overheats and the viscosity drops rapidly (inside the bearing).
I am suspecting that the cooler and plumbing after the cooler is causing the oil flow to the engine to drop so low that there is not enough oil flow through the bearings to prevent viscosity breakdown, above 4K rpm. The pressure drop you are seeing is the oil viscosity dropping in the bearings. If I am correct, going to a thinner oil should improve the situation, because you will get more flow through the cooler an plumbing after it, and thus more oil flow to the engine.