A jalousie unit is a window or door made up of parallel glass slats, held together in a frame. The slats are adjustable, allowing for ventilation and light control. The term 'jalousie' comes from the French word for 'JEALOUSY, ' as these windows were originally designed to allow people to see out, but not in. The four-letter solution to this crossword is 'SLAT, ' which refers to the individual glass pieces that make up the jalousie unit.