This is a common (though not necessary) way to refer to any object with multiples of a given component.
For instance a three wheeler vehicle which can be a car, bike (strange in itself as 'bike' is short for 'bicycle' or 'two wheels'), handcart or anything else with a tricycle wheel arrangement.
Also a sailing ship with three masts can be called a three master
A 'hackney carriage' with four wheels was called a 'four wheeler'. They are often mentioned in the Sherlock Homes books.
A truck with six wheels (and sometimes one with ten wheels on three axles) is also called a 'six wheeler'. This is also extended to 'eight wheeler', 'ten wheeler' (another name for a six wheeler with twin wheels on the rear axles) and so on.
Those are just a few of the common names for things that are named in the same way as 'four poster' beds so you see it's quite old: at least Victorian, Conan Doyle was writing contemporary fiction in contemporary language when he wrote the Sherlock Holmes books.