#include <bits/stdc++.h>
using namespace std;
long long n, temp, ans, t, k, d, temp2;
int main() {
  cin >> n >> t >> k >> d;
  temp = (n * t);
  long long timereq = temp / k;
  if (temp % k != 0) timereq++;
  bool ok = false;
  temp2 = 1;
  while (true) {
    if (temp2 % t == 0) {
      if (ok) ans += 2 * k;
      ans += k;
    }
    if (temp2 == d) ok = true;
    if (ans >= n) break;
    temp2++;
  }
  if (temp2 < timereq)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
  return 0;
}
