#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, t, k, d, T;
  cin >> n >> t >> k >> d;
  if (k >= n)
    cout << "NO" << endl;
  else {
    T = (((n - 1) / k) * t);
    if (T > d)
      cout << "YES" << endl;
    else
      cout << "NO" << endl;
  }
  return 0;
}
