JASON A 4-post bed. The kind of old fashioned beds where each corner of the wooden bed frame has a 6-foot tall wooden post. Some people hang curtains around the top if you connect the posts together with rods, for decoration and privacy when you let the curtains down. In tropical countries people drape a thin see through mosquito netting so you can sleep all night in a personal protection zone free from annoying insects and still get the cooling ocean breeze with your windows open. People who live in the tropics by the ocean dont put air conditioning in their houses. Some people leave the posts plain and bare as some nicer, expensive 4-poster beds are made from beautiful wood with fancy knobs of endless designs at the top of each post, and designs in the posts themselves that match the headboard and footboard. You can buy some nice 4-poster beds at Home Depot even.
Erika It is the beds that they sit on, in one of the books Dumbledore comes into Harry's bed room and says "I've never really liked these four-posters, I light mine on fire in my fourth year." While touching and looking at Harry's bed.