#include <bits/stdc++.h>
using namespace std;
int main() {
  double n, t, k, d, n1;
  cin >> n >> t >> k >> d;
  n1 = ceil((n / k)) * t;
  bool b = false;
  double cakes = (d / t * k), y = d;
  while (cakes < n) {
    y += t;
    cakes += k * 2;
    b = true;
  }
  cout << (y < n1 && b ? "YES" : "NO") << endl;
  return 0;
}
