The Collaborative International Dictionary of English v.0.48:
Four-poster \Four"-post`er\, n. A large bedstead with tall posts at the corners to support a canopy or curtains. [Colloq.] [1913 Webster]WordNet (r) 3.0 (2006):
four-poster n 1: a bed with posts at the four corners that can be used to support a canopy or curtains