It looks like Sonos is about to launch another new speaker, having already unveiled the portable Sonos Roam, the Symfonisk Picture Frame speaker created in collaboration with Ikea and the Sonos Beam (Gen 2) this year. Sonos has not confirmed anything yet, but it has updated its app to reflect an upcoming product.
As the screenshot below shows, the 'Sub Mini' has turned up in the Sonos app. Unfortunately, the app does not reveal many details about the Sub Mini. However, it states that the subwoofer cannot be used as a second subwoofer, a feature supported by the Sub Gen 1, Gen 2 and Gen 3.
Additionally, Sonos describes the Sub Mini as 'a smaller, cylindrical subwoofer' rather than the square subwoofers of the three Sub generations so far. Presumably, the Sub Mini will be a smaller alternative to the Sub Gen 3, which measures 40 x 40 x 15 cm and weighs 16 kg. The Sub Gen 3 also costs US$749, so we would expect the Sub Mini to come in for a bit less too, especially considering its lack of Second Sub functionality.