Too little flow left over at # 4 to feed the rods enough. Doesn't matter what the gauge says, if it is leaving the main bearing so fast there isn't enough to feed the rods, they starve. I would restrict the passage to the rocker shafts, or feed them externally to keep max oil to #4 main. Also while the motor is down, check clearances on the lifter bores and lifters themselves and any other place oil can bleed off (cam bearings?)due to wear.