No, the Sub 4 cannot be bonded with Play:5 (Gen2)s when they are a single unit or stereo pair.

In addition, if the Play:5s were being used as surrounds with the Arc, you are unable to dual sub a Sub 4 with a Sub (Gen2)

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

Thanks

But, the Sub Gen 2 could be bonded to the Play:5 Gen 2 stereo pair, and a Sub Gen 4 could be bonded to the Arc. 

Shuffling Subs around isn't difficult and I'd rather buy a Sub 4 and rearrange for compatibility than buy an older Sub.

I’d rather buy a brand new Sub (Gen3) for almost half the price of a Sub 4 whilst also giving greater flexibility over use.

I'm more interested in making sure my main listening areas are going to see any future features Sonos releases that don't appear on older devices.

Older gear (new, refurbished or used) is cheaper but I'm not sure it is the best value long term.

Individual choice call, so go either way based on your needs.

Considering the Sub (Gen3) wasn’t listed on the recent update on “older devices” that’s not something to worry about in the foreseeable future if making such a purchase.

On the other hand the Sub 4 is shown to have more memory and processing power.

And yet it still plays the same bass for double the price…

Now.