Some of the design details on Rolex watches can help you distinguish between the 4, 5 and 6-digit Rolex reference numbers. For example, the type of luminescent material used on Rolex watches has evolved over the years.
If a watch uses radium to emit light, it is guaranteed to have a four-digit Rolex reference number. Because of the longevity of tritium, it can be slightly tricky to determine its condition, but if a Rolex replica watch has tritium luminescence, then it can be assumed to have a four- or five-digit reference number. Similarly, if the particular watch in question has luminescence or superluminescence, then it is more likely to have a five- or six-digit reference number. Finally, suppose a watch has Chromalight, which glows blue. In that case, it must have a six-digit Rolex reference number, as this material itself only appeared after Rolex had almost completely switched to a six-digit reference number system. 
Rolex replaces the dial and hands with newer versions when a watch needs servicing. While you won’t find four-digit models with blue Chromalight luminous dials, you will find some with Luminova or Super-Luminova. You will appear similar to the luminosity that can be observed on later models with five- or six-digit reference numbers material style.
Rolex has also changed the type of crystal used to protect its watch dials. Initially, the company used acrylic crystals, but now it uses clear sapphire crystals. This shift began slowly in the 1970s, picked up pace in the 1980s, and was completed in the early 1990s. With the exception of the Rolex Quartz 5100, Datejust 1630 and Rolex Date 1530, almost all four-digit Rolex reference number replica watches were originally fitted with acrylic crystals. In addition, all Rolex watches with six-digit reference numbers have sapphire crystals. That said, depending on the specific model, five-digit Rolex reference numbers can have either acrylic or sapphire crystals.
In short, the evolution of the fake Rolex reference number from four to five and then to six reflects the evolution of the watch itself. Generally speaking, the longer the reference number, the newer the watch, and each generation of different models have its own unique characteristics and features.
