The term 'cluster bean' is a common name for the plant Cyamopsis tetragonoloba, which is a member of the legume family. It is an annual herbaceous plant that is native to India and is widely cultivated in tropical and subtropical regions. The plant is used as a vegetable and for fodder, and its seeds are used to make guar gum, which is a thickening agent used in food products. The four-letter solution for this crossword is GUAR.