import java.util.Scanner;

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