Rod bearing noise will go away at idle if you pull the spark plug wire on that cylinder, valve train noise won't go away.
The rod bearing noise comes from the gap(worn bearing) at the connecting rod and crank journal, crank pushes rod and piston up on compression stroke and then passes top dead center which opens the gap, spark plug fires and the piston and rod are pushed down forcefully which makes a Knock noise when it closes the gap on the crank journal.
Worn rod bearing won't effect oil pressure much.
So if you remove the spark, you remove the noise, yes engine will miss, but you are just seeing if noise goes away.
With engine off pull off one spark plug wire at a time from the coil pack, and put each back on but not tight, start engine, now using RUBBER handled pliers pull off 1 wire at a time, repeat for all wires, if noise stays you have valve train noise.
Main bearing is a different animal altogether, it will reduce oil pressure quite a bit, and outside of tracking the knock down with a broom handle(my tool) there is no real test for it except as you said, a knock and low oil pressure.
Busted lifters are not uncommon on the 4.0l, unfortunately you have to remove the heads to get at all the lifters.
Ford fuel injectors can be quiet loud, tick, tick, tick
Rattling could be a few things, when engine is cold loosen fan belt and take it off crank pulley, start engine, see if you still hear the noise, cold engine can run 2 minutes without water pump, Battery light will be on(no alternator), raise RPMs as well, without fan noise it may be easier to hear where it is coming from.
Around 1986/87 Ford removed the oil pressure sender and replaced it with an oil pressure switch.
Switch is set for 6psi of pressure, under 6psi switch is open(no ground), over 6psi switch is closed(grounded to engine)
Oil pressure gauge has a resistor, when key is on, engine is off, gauge shows 0, when engine is running at oil pressure is above 6psi gauge is grounded and will show just below 1/2 usually.
Any variation of the needle, besides going to 0, is a voltage fluctuation not a change in oil pressure.
I think you can change a '93 to a "real oil pressure" gauge, by swapping the oil pressure switch for a PS60 oil pressure sender, and then pulling out the instrument cluster and by passing the resistor, it was added to the back of the gauge.
On later models it is part of the gauge so can't be by passed without replacing the gauge
Have a read here: "Dummy" Gauge