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