import java.util.Scanner;

class main{
    public static void main(String args[]){
	Scanner scan = new Scanner(System.in);

	int H = scan.nextInt();
	int W = scan.nextInt();
	int N = scan.nextInt();
	int K = scan.nextInt();
	
	int S = H * W;
	int lose = 1;

	for(int i = S; i > 0; i--){
	    lose++;

	    if(lose > N){
		lose = 1;
	    }
	}

	if(lose == K){
	    System.out.println("NO");
	}
	else{
	    System.out.println("YES");
	}

    }
}