@AAMfP For e.g. the Markdown backend, the columns are completely user-customizable.
The screenshot shows the CalDAV backend though, which is indeed fixed to 3 columns, yes.
It used to have a fourth column for the backlog, but from day-to-day usage I found it more convenient to have the backlog as low-prio items in the todo column. They're off-screen, at the bottom of the list. Otherwise, lots of screen space is wasted on items I don't care about. This would eventually be customizable though.