That’s odd. I’d be tempted to disconnect my controller from the Sonos system, wait a minute, than reconnect it, to see if that made a difference. 

I note that here in the US, there was a controller and firmware update, although I haven’t explored them yet. Have you applied them?

I removed all speakers from my system and added them back to the new 5Ghz network. No change. SUB4 still doesn't have the arrow pointing to the side. From what I can see, my software is up to date and I haven't seen any controller updates.

I had a similar issue and the cause was one of my portable speakers, that was in deep sleep mode, did not have the latest update.

Have you tried removing the Sub that DOES have the arrow and see if the Sub 4 gets the arrow?

yes, exactly now, nothing has changed :(

​@addriano 

• Sign out of your account.
• Delete the app.
• Restart your phone.
• Reinstall the app. 
• Log back into your account. 
• Go to Settings/General Settings/System Updates/Check for Updates (and run any that appear - yours appears to be on an old build - the latest is 84.1-64070).
• See if your Sub 4 appears with an arrow.

I did as you wrote and now I have version 84.1-64070. Still the same, nothing has changed.

Frustrating. Can you try installing/opening the app on another device? You could try the desktop app for Windows or Mac, downloadable from the Sonos website, and see if the Sub 4’s options appear there, under Settings and then Living Room.

https://support.sonos.com/en-gb/downloads

It might help establish if it is fundamentally missing, or if it’s only on your phone...

​@addriano 

UPDATE: It is entirely possible that they are controlled together, as one. I have read that this is the case for dual Subs in a Sonos system. Maybe someone can confirm or deny, but you may just have one set of EQ controls to adjust BOTH subs together...

I've also heard that I can't control the EQ of two SUBs separately. But the data that we can see by expanding the arrow doesn't change anything anyway. It's just informational data. That's why I don't think it has any significance. But maybe it does, let someone from SONOS comment.

The additional connection data has never been available for the Sub 4.

It’s just another bug with the app that has yet to be fixed.

Does the arrow next to the existing Sub bring up details (e.g. serial numbers) of both subs on the next screen, or just the single sub...?

only the single sub