Ignoring the room size, just for one moment… if you intend to ‘bond’ the Amp/Ceiling Speakers to the Sonos Arc & Sub as a Home Theatre setup, then when watching either a DD 5.1, or an Atmos TV Movie, the Amp in that configuration, is set to play the rear left/right Home Theatre channel audio only - It will not play any front channel audio.
Also, when playing either stereo, or Atmos, music audio, then by default the Amp/Speakers play just ‘Ambient’ audio for ‘stereo music’, involving the main Arc soundbar, …or the Amp/Speakers are used for rear/side channel audio for Atmos music… So, with those things in mind, you should really stick to just two speakers only, set to the left and right, above or behind, your usual main seated/listening position in the room.
If you play the same ‘rear’ channel audio through 4 ceiling speakers, you will be doubling their rear-channel output and muddying the audio intended for a surround sound, or Atmos audio, Home Theatre setup, particularly if any of those speakers are located out in front of the usual main seated/listening position in the room
I personally would not do this, if the main purpose is to use the bonded setup with 2 or 4 speakers as a TV Home Theatre if any of the speakers are to be position in front of the main listening position. At best I would suggest 2 speakers only set either above, or behind, the main seated/listening position.
Even then, I’m personally not a fan of using any ceiling speakers at all for use with a Sonos Home Theatre setup in a room.
You may want to perhaps consider using two Sonos speakers instead, like Era 100s or 300’s for the Home Theatre setup and then using just two in-ceiling speakers for the Amp, just for (background) music audio only — or if you have not yet purchased the Amp/(ceiling) Speakers maybe even forget about those altogether.
