#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1.0);
int n, t, k, d;
int main() {
  cin >> n >> t >> k >> d;
  int cnt = (d + t - 1) / t * k;
  int rem = n - cnt;
  if (rem > 0) {
    if (d % t == 0) {
      if (rem <= k)
        cout << "NO" << endl;
      else
        cout << "YES" << endl;
    } else
      cout << "YES" << endl;
  } else
    cout << "NO" << endl;
  return 0;
}
