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