#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long n, i, j, q, r, g, x2, d, m, e, h, tt, s, l, z, x, y, x1, y1, z1, k,
      t, p;
  long double sg, fg, d1, sig, nd;
  string a, b, c;
  cin >> n >> t >> k >> d;
  m = (n / k) * t;
  if (n % k > 0) m += t;
  e = 0;
  r = d;
  while (n > 0) {
    while (e < r) {
      n -= k;
      e += t;
    }
    if (n > 0) {
      n -= k;
      r += t;
    }
  }
  if (max(r, e) < m)
    cout << "YES\n";
  else
    cout << "NO\n";
  return 0;
}
