package yukicoder;

import java.util.Scanner;

public class N384
{
	public static void main(String[] args)
	{
		Scanner sc=new Scanner(System.in);
		long H=sc.nextLong(),W=sc.nextLong(),N=sc.nextLong(),K=sc.nextLong();
		long buf=H+W-2;long ans=0;
		if(buf==0){}
		else
		{
			if(buf%N==0){}
			else{ans=buf%N;}
		}
		if(ans+1==K){System.out.println("YES");}else{System.out.println("NO");}
	}

}