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.