#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, k, d, cnt = 0;
  cin >> n >> t >> k >> d;
  int t1 = 0, t2 = d;
  cnt = (n / k);
  if (n % k) cnt++;
  for (int i = 0; i < cnt; i++) {
    if (t1 <= t2)
      t1 += t;
    else
      t2 += t;
  }
  if (max(t1, t2) < cnt * t)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
  return 0;
}
