Short answrer: it shouldn't connect to the Amp. At least not directly. The Virgibn box connects to your TV, The TV is connected to the Arc and the Arc connects wirelessly to the Amp. Wghen used for surround speakers The Amp connects via a direct 5Ghz link to the Arc, when the Amp is stand alone (has it's own “room” in de Sonos app) it will connect via pour wifi or Sonosnet.

In a surround set up the Amp will power the rear speakers, that should preferably be at ear height behind the listener, besides that the sound channel accounts for two surround speakers. Using four speakers placed high up could work, but is not a preferred set up.

What are you going to be playing through the four In-ceiling speakers?

Unless something has changed recently there is no routing of overhead channels from the Arc to the Amp, any speakers connected to the Amp would be Surround channels which you would not want in your ceiling if you are trying to create an Immersive envelope for Atmos.