Good question. I've never weighed any of the hundreds of freewheels I've worked on.

Of course there are two variables in every freewheel's weight; the body and the cogs & spacers. Bodies can vary significantly. For instance, without seeing a picture of your two freewheels, I'd guess the 4 gram difference is primarily in the bodies. Regina bodies tend to use less steel than say a Suntour, Shimano, or Sachs.

For instance, look at this Regina Corsa. Originally this design was for a 4 speed freewheel. Regina then determined how to thread the two smallest cogs together, adding the fifth cog. Eventually this same model could thread the three smallest cogs together without enlarging the body, making it a 6 speed.



But now look at the progression in Suntour Perfect bodies. These three pictured bodies are all Perfects. L to R: 6 speed standard, 6 speed Ultra spaced, 5 speed. The difference in body size is apparent.



The second variable would be how much the cogs and spacers weigh. The oldest models tended to use steel spacers. Newer models almost exclusively use plastic spacers. In regard to the cogs, older cogs tended to be solid with no cutouts, newer cogs usually have more drillium on the cogs.