I don’t have a Sub 4 but own all 3 of the previous generations. I have zero intention to currently replace an older Sub with a Sub 4.
I hear no difference between the previous generations and as Sonos allow a Sub (Gen3) and Sub 4 to be used in a dual Sub setup, I doubt there is any tangible sound difference between them. I’ve not seen any review either that recommends replacing an older Sub with Sub 4.
As @buzz mentions, with the Arc Ultra you cannot dual Sub the first Sub gen with Sub 4, so it would be a direct replacement. I cannot see any compelling reason why someone would do that, if they didn’t then use redundant Sub in another Sonos room, so the reason for purchase is really to have a Sub in another room, rather than improve the sound on the home theatre setup.