I guess that the jury is out until #4 comes off. Usually with a knocking bearing, you will see some clues on the culprit. You can buy .001" bearings and you could buy one pair for fitting.
A low pressure motor will have dippers on the rods and dipper troughs. Also, a badly worn main bearing can contribute to low oil pressure.
What was the knock like? A rod bearing knock is harsh and metallic and is present as the RPM starts to drop when you rev the engine a little. This is where there is no load on the bearing during part of the cycle. A main bearing knock will be there at low RPMs and definitely when under load.
