Usually the 4B was a year or so behind the 6Bs but not always. A lot depended on emissions requirements and the application.
So in order to understand why things got quieter, you gotta understand how the diesels worked. Both the VE injection pump of the 89-93 & the P7100 pump of the 94-98.5 were mechanical. The VE pumps injected fuel at a lower PSI where as the p7100 hit it with a LOT MORE. Diesel fuel regardless of the pump is injected at several tens of thousands of psi. The higher the injection psi, the bigger and hotter the boom. Which then adds to the noise. The 89-93s are much much quieter than the 94-98.5.
Then in 98, roughly half way they the model year, the epa requirements forced the introduction of the 24 valve using the VP44 injection pump. This pump tried to combine timing, volume and PSI. This made things even louder as now you had twice the lifter hits and just as cam over lap affects sound and lope of a V8 gasser, those three things inside the pump made things lope and do so loudly. One thing people run into when turning up a VP44 is if you go too far with timing or volume, you can get a shuttering effect on the fuel. Not a good thing.
in 2003, Cummins switched to the CP3 pump, which supplied a single rail injection system at a constant And consistent injection psi. That’s all it did. The injectors were then controlled directly by the ecm. Which controlled the pulse and timing, this adjusting the volume and timing at the cylinder instead of an injection pump. This greatly eliminates psi variations and quieted the injectors and the subsequent ignition down. They also changed out lifters and upgraded valve springs and added valve spring dampeners. All those things resulted in the quietness compared to the 90s Cummins.
If one does swap to the mechanical p7100 pump in a 03+, the noise level will increase, but it will be somewhere between that of the first gen Cummins and the common rail Cummins and no where near as loud as the VP44 era engines
