I have had a Sub 4 and Sub gen 2 in the same room, hooked to different Rooms and I couldn’t hear a difference, my ears aren’t great but that is mostly at higher frequencies.

:-) Get the new Sub, tell yourself it was necessary, then put the old one in another Room that lacks one. I convinced myself, the spouse isn’t buying my tale though.

If you want to add a second SUB to a surround room, note that ARC ULTRA does not support SUB V1 as a second subwoofer.

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.

1. I wanted another Sub (that wasn't really justified) to hook to my Era-300 pair.

2. I had a discount that worked for any single item. The soundbar sets count as one item.

3. The spouse said as long as it fits in YOUR room.

I wouldn't recommend updating, makes no Sonos.

But you can never have too many (well 32) Sonos.