You can only add two surround speakers to a Sonos home theater setup. If you use the Amp as surrounds with the Arc, the two speakers wired to the Amp will play the correct left and right surround audio.
You could wire four speakers to the Amp, but you would get the same left surround audio playing out of two speakers and the same right surround audio playing out of the other two speakers.
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.
Maybe the Sonos in-wall speakers would be a possibility if there is no place for a regular speaker?
The Era 300s are going to give far the best sound and Atmos support, the Era 100s would be pretty good, the in-wall not bad and the ceiling ones my last choice.
In-wall $650 https://www.sonos.com/en-us/shop/wall-speaker-pair?sonos=native
Era 300 $850 https://www.sonos.com/en-us/shop/immersive-music-set-era-300-pair
Era 100 $470 https://www.sonos.com/en-us/shop/two-room-set-era-100
A contrarian configuration if you want to use the ceiling speakers for surround: Add a Volume control for each pair of ceiling speakers. When you want surround, mute the extra pair of ceiling speakers and use only the rear most pair. When playing music both pairs can be used.
A speaker selector switch at the Amp would also work to disable the too far to the front speakers. Might be cheaper than the volume control.
Still would be my last choice for TV but if you also want background music too it makes more sense.
If being cheap is the goal, a single switch, costing a few bucks at most, is all that is needed to mute the unwanted pair when watching TV. Volume controls allow setting the most appropriate level in each area.
