If you are primarily listening to music, I would just add the Sub. But if you want a Sonos home theater setup, a Sonos Arc with a Sub is a great option. You will be able to group the four ceiling speakers with the Arc, but TV audio will be slightly delayed on the ceiling speakers. Streaming music will be in sync though.

You could add the ceiling speakers as surrounds to the Arc, but more than likely they aren’t positioned correctly to function as rear surround speakers. And Sonos only recommends using two speakers as surrounds with the Amp, not four.

Soundbar is usually associated with a TV or projector. Is supporting video your objective? Are individual connections to the four ceiling speakers available at the AMP location?

Tv and we stream music. 

Yes all 4 speakers are hard wired through the walls at the location of the Sonos amp. 

Are the ceiling speakers currently being used with TV audio?

Yes connected to Sonos amp. I also stream music through them. 

A soundbar would result in better alignment between picture and audio. Unless your TV is close to the ceiling and speakers, the picture and sound do not currently coincide.

“Best” is a personal feeling. If the current alignment is good for you, adding SUB would be a step up. If you want the alignment, adding ARC and SUB, then using only two ceiling speakers as rear surrounds will provide a more theater-like TV presentation. Yes, I know that there will be two unused ceiling speakers and this might chafe your sensibilities. One relatively easy work around for music would be to add a switch to re-connect the unused speakers when you are listening to music.