We don't have synonyms for four-posters yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are four-poster, four posters and four poster.
(n) :
A four-poster bed, which has four high posts, one at each corner, supporting a canopy.
(adj) :
(attributive, of a bed) Having four vertical columns, one in each corner, that support a tester, so that curtains can be hung to exclude draughts from the sleeper.
(n) :
A four-poster bed, which has four high posts, one at each corner, supporting a canopy.
(adj) :
(attributive, of a bed) Having four vertical columns, one in each corner, that support a tester, so that curtains can be hung to exclude draughts from the sleeper.