I think this itch page is abandoned, you can get the 4.0 for free on patreon, and the update go as far as 3 weeks ago with 4.1.
Viewing post in Debauchery in Caelia Kingdoms comments
It's clearly not abandoned since the dev responded to someones comment three days ago (as of March 26). It's entirely possible that this Itch page intentionally offers an older version so that Patrons have access to a newer version than the free-to-players, so as to offer more incentive to pay.
A lot of game devs do this, sorta like how the free version of Ravenfield is a much older version than the paid Steam version.