An architect is designing a new windmill with four sails. In his ​sketch, the​ sails' center of rotation is the​ origin, (0,0), and the tip of one of the​ sails, point R ​, has coordinates (2,4). He wants to make another sketch that shows the windmill after the sails have rotated 90 degrees about their center of rotation. What would be the coordinates of ​R'?