Easier just to log in to your Sonos account online and view the Sub gen in your list of devices

https://www.sonos.com/en-gb/myaccount/system/households/devices

This is from the FAQ’s for the Ultra:

Which generations of Sub are compatible with Arc Ultra?

All generations of Sub and Sub Mini are compatible with Arc Ultra. A dual Sub setup requires Sub 4 and/or Sub (Gen 3).

So either of your Subs will be fine, unless you mean  bonding them both? 

One of your existing Subs must already be a Gen 3 as you have a dual Sub setup in Living Room from your screenshot.

Yup, but the other also needs to be Gen3 or Gen4 in a dual-Sub setup with Ultra. You’re right though, that only one needs to be replaced if one is Gen3.

Unless they don’t want to mix a gloss and matt finish.

Sorry that screenshot was from the old set up with the subs bonded to an Arc (not arc ultra)

I have just set up the arc ultra and bonded it with the 2 new sub generation 4

so the two on the original post are now “unused”

and those are the ones I’ll be looking to sell

Thanks problem sorted (one gen 3 and a gen 2…who knew)