Even if you have both devices connected with Ethernet cables, I’d suggest leaving on the WiFi. The Sonos will ‘pick’ the fastest response time, which (perhaps oddly) should be via the 5Ghz WiFi connection, and not through the router. Unless you’re having transmission issues between the Arc and the Sub, I’d even remove the cable to the Sub. Chances are fairly slim it is in use. 

Bruce, thank you for your response.  Understand your comment that WiFi may be better than a hard-wired connection.  And that it’s better to let the system/Sonos decide which connection.  That being said, for Sub (Gen 3), we/the user had the option to force the system to use Ethernet by disabling WiFi.

My question why is the option to “disable WiFI” no longer available for Sub 4.

You might call Sonos support to ask that,  they haven't shared that with anyone I know.

I was surprised to see the Ultra and Sub 4 still had Ethernet ports, the new Era line doesn't. 

I’d also like to know the answer to this. There’s an ethernet port there. I connected a cable to it. I’d like to know if it’s being used or not, or to force it to be used to free up wifi for other things in the house.

People in this and other Sonos forums constantly reduce people’s issues to their sucky wifi. One of the reasons I like this ecosystem are the ethernet ports…

Let me decide this. Give me the option.

The connection between a soundbar and sub is an ad-hoc private network.  It’s not using up any bandwidth on the regular network, so you wouldn’t be “freeing up” anything. 

Check About My Sonos System… .

I don’t even have a soundbar. I had a Sub3, now I have a Sub4 and the option seems to be gone. I want to know what it’s doing. My router seems to think it’s using ethernet atm. Good I guess. 

EIther way, it’s the same connection.