using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace foryuki
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] a = Console.ReadLine().Split();
            long H = long.Parse(a[0]);
            long W = long.Parse(a[1]);
            long N = long.Parse(a[2]);
            long K = long.Parse(a[3]);

            Console.WriteLine(((H * W - 1) % N + 1 == K) ? "YES" : "NO");
        }

        //-------------------------------------------------------------

        static int[] ConvertStringArrayToIntArray(string[] array)
        {
            return Array.ConvertAll(array, str => int.Parse(str));
        }

        static List<int> ConvertStringArrayToIntList(string[] str)
        {
            var list = new List<int>();
            foreach (var c in str)
            {
                list.Add(int.Parse(c));
            }
            return list;
        }
    }
}