#include <bits/stdc++.h>
using namespace std;
int n, t, k, d, kake, time1, time2;
int main() {
  cin >> n >> t >> k >> d;
  for (int j = 0; j < n;) {
    j += k;
    time1++;
  }
  for (int i = 0; i < n;) {
    i += k;
    time2++;
    if (time2 * t > d) {
      i += k;
    }
  }
  if (time1 > time2) {
    cout << "YES\n";
  } else {
    cout << "NO\n";
  }
  return 0;
}
