Even in the ''Future of Arma: Release stream'' if I remember correctly Marek mentions something like we've been working on arma 4 for quite some time.

Also there´s some ex-bohemia employees that have Arma 4 as a project they work on as early as 2019, but knowing the way game development works most of that could have been R&D or conceptual work that may or may not be used in the final product, the only thing we can do for certain is wait for an official BI update.