From what I have read only the Sub3 and Sub4 can be used as 2 woofers with the Arc Ultra.

Perhaps see this Sonos Support link:

https://support.sonos.com/en-us/article/set-up-your-sonos-sub

It’s surprising how much info Sonos provide in a product’s FAQ’s:

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).

It’s also confusing. The sonos support link from @Ken_Griffiths says: “

• Sub (Gen 1, 2, and 3) are compatible with all Sonos home theater speakers.
• Sub 4 is compatible with all Sonos home theater speakers except Playbar and Playbase”

That sounds like I could combine the gen 2 and 4 together since “Sub 4 is compatible with all Sonos home theater speakers...”

So to be clear: The answer is NO you cannot combine a sub gen 2 with a gen 4? 

Edit: Apparently yes. Oddly, the Arc, which I already have, would let me use a gen 2 with a gen 3 so maybe in a future update….

The piece you’re quoting in your post here, is referring to a single ‘standalone’ Sub with a Home Theatre Soundbar (including the Arc Ultra) - things change quite a bit when two (Dual) Subs are used with the Arc Ultra. It is explained in the support article under the heading "Using two Subs with your home theater speaker”.

It is also open to interpretation as the FAQ wording is inadequate.

In what way is it inadequate or open to interpretation?
Any single Sub or Sub Mini will work with Arc Ultra. Hence they are all compatible. But, if you want a dual Sub config it needs one of them to be Sub 4, and the other to be Gen 3 or 4. 

Good lord. Clearly you get this stuff more than I do, which is why I asked the question, so let’s not start some internet fight. The question has been answered so we don’t need to continue.

All previous dual Sub setups have required at least one Sub (Gen 3). The FAQ statement could easily be interpreted as only requiring either one Sub 4 or Sub (Gen 3) for a dual Sub setup.

Given the number of posts enquiring about what dual Sub combinations will work with Arc Ultra is further evidence the FAQ wording is inadequate.

Under the link Ken provided earlier, Sonos state for connecting two Subs:

• Arc Ultra: Compatible with two Sub 4s, Sub (Gen 3)s, or one of each.

That is how the dual Sub setup should be explained in the Arc Ultra’s FAQ.

My apologies. It’s not so much “open to interpretation” as “contradictory”! The FAQ says, in effect, “at least one must be a 4, but the other could be a 3 or a 4”. The other guide says you can use two Gen 3’s. 

I wouldn’t say it’s contradictory, you and I both know what it’s trying to say, it could just be made clearer.