import java.util.Scanner;
public class Main {
	static int[] an = new int[1000000];
		public static void main(String[] args) {
			Scanner sc = new Scanner(System.in);
			long H = sc.nextLong();
			long W = sc.nextLong();
			long N = sc.nextLong();
			long K = sc.nextLong();
			if((H + W - 2) % N + 1 == K) System.out.println("YES");
			else System.out.println("NO");
		}

		

}