#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;

int h, w, n, k, x;

int main() {
  cin >> h >> w >> n >> k;
  x = (h * w - 1) % n;
  cout << (x == k - 1 ? "YES": "NO") << endl;
  return 0;
}