Just as a note--pistons 1 and 4 move together as a pair--2 and 3 as a pair.
If your motor is quieting down after warm-up I highly doubt you have a rod bearing problem. Piston slap will diminish gradually as the piston expands on warm-up. A rod bearing that's going may rap on start-up but once it's truly "gone" it will continue to rap - especially when accelerated -- and at that point the motor isn't long for this world. My near 300k mile '80 B21F sounds like a diesel on start-up--quiets down to normal after a drive (or maybe my hearing is shot from driving this noisy beast). -- Dave