[Mike Taylor 3601]

if crank wasn't magnafluxed when ground, may have been cracked and then broke.
#4 main bad and others good sounds like crank was cracked and rubbing bearing before it broke, if look closely may see where #3 and # 5 main bearings have been rubbed some,most of the time when you have a few main bearings rubbed/wiped out the crank will be cracked and flexing.
Was the radius left in crank when reground?If had 1/8 radius and it was cut less or square that weakens the crank also.
When I tear into engine with broken oil pump,I check balance of rotating assembly,clearance between oil pump shaft and block and oil pump pick up/pan clearance.
but there is chance since most bearings were OK that crank broke and vibration from that broke oil pump.
Mike Taylor 3601,