Oh. Well, So they would need a custom algorithm to downmix 5.1 to 4.1 channels, because it is not standard and widely established in the way that downmixing from 5.1 to stereo is, and not accounted for when mixing movie soundtracks. And no, the simple math of adding 50% of the center channel's level to each of the front left and right channels while keeping SL and SR as they are would not give you the result the movie soundtrack mixer (re-recording engineer) had in mind.As the team said 4.1ch, there is no center speaker.
Edit;
(From now on my fantasy. )
I think it will probably be their first step. Depending on user usage and demands, the number of channels may increase in the future.
However, in reality, it is possible that one WiiM will be required for each SR1ch. In that case, 4.1ch requires one WiiM Ultra (or WiiM amp pro) and two WiiM mini (or Pro and higher devices).
Anyhow, this seems a very niche application for WiiM devices for now, and so far one shouldn’t expect them to replace AVRs.