#include <bits/stdc++.h>
using namespace std;
int main() {
  double n, t, k, d;
  double a, b, c = 0, q;
  cin >> n >> t >> k >> d;
  a = ceil(n / k);
  if (floor(n / k) == 0) {
    cout << "NO";
    return 0;
  } else {
    b = a * t;
    for (int i = 1;; i++) {
      c = d + t * i;
      q = i + floor(c / t) * k;
      if (q >= a) break;
    }
    if (c < b)
      cout << "YES";
    else
      cout << "NO";
  }
  return 0;
}
