4 Homepods in one room

Can they all play together ?

You’d have to set up two stereo pairs, then select both pairs in AirPlay. The pairing might look like this, depending on where the seating / listening position is. It is definitely not going to be a quadraphonic experience and might sound strange.

Alternative is to not pair and just select all four speakers in Air Play, but the space-adaptive technology in the speakers will not be optimal.

I’ve got one room with six smart speakers in it. My big kitchen/living room/office has a stereo pair of Sonos Play:1s at my desk, a Sonos Beam and a pair of Play:1 surrounds on the TV, and a HomePod in the kitchen. This setup is fairly new to me (the Beam and HomePod were Black Friday purchases) but in my limited experience so far I like it. With AirPlay 2 I can play the same music in sync from everywhere. With this many speakers, the music basically fills the room, without sounding like it comes from a particular source. However, when I sit down at the desk I do get a sense of stereo separation (though perhaps not as strongly as if the desk speakers were the only thing playing).