I honestly don't think it's bearing issues. If you had loose clearances, the
oil pressure wouldn't just dip at 4300 rpm. You did check a couple right? If they looked new, chances are that you don't have a problem with the others.
I think you have a problem with the pickup, or the
oil is not returning as it should.
If I would have been checking this, I would have bypassed all the remote filter junk and the cooler altogether. Slap a filter directly to the block, even if it is a shorty, and then see if the pressure dips.