 A broad area of low pressure formed within the wave the next day while located 260 mi (420 km) southeast of the Cape Verde islands.Though convection decreased early on August 21, the area of low pressure remained well-organized and the system developed into Tropical Depression Four late on August 21.Water temperatures remained warm enough for development, while upper level shear was minimal as the depression moved west-northwestward due to a ridge of high pressure to its north.