I don’t have a Sub 4, but by all accounts there is marginal difference in bass produced when comparing Sub (Gen3) and Sub 4.
The fact Sonos allows a dual Sub setup incorporating both Sub (Gen3) and Sub 4 further demonstrates that there is no dramatic sound difference between the two Sub versions, otherwise they wouldn’t allow it to be supported configuration.
A dual Sub setup can be beneficial as it can even out the bass response in a room. However, if you’re happy with the performance of your single Sub (Gen3) as it is, there’s no compelling reason to purchase a Sub 4. I certainly wouldn’t replace a Sub (Gen3) with a Sub 4 at present. I’d rather spend the funds placing speakers in other rooms.