Four. According to the information of the inquiry brush question, a pool of water in the brush question belongs to a mathematical type question, and the answer is that if you buy a sweet cone, at least four pieces will be paid in RMB of 50 cents, because 5 plus 5 plus 5 plus 5 equals 20 cents equals 2 yuan, then at least four pieces of 50 cents will have to be paid.