#include<bits/stdc++.h>
using namespace std;

int main(){
  long long h, w, n, k;
  cin >> h >> w >> n >> k;

  if(w == h){
    if(w % n == k){
      cout << "NO" << endl;
    }else if(w % n == 0 && n == k){
      cout << "NO" << endl;
    }else{
      cout << "YES" << endl;
    }
  }else{
    if((w + h - 2) % n == k){
      cout << "NO" << endl;
    }else if((w + h - 2) % n == 0 && n == k){
      cout << "NO" << endl;
    }else{
      cout << "YES" << endl;
    }
  }
}