#include <bits/stdc++.h>
const long long N = 1e5 + 5;
const long long mod = 1e9 + 7;
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(NULL);
  ios_base::sync_with_stdio(0);
  cout.tie(NULL);
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int sT, dT, temp;
  sT = ((n + k - 1) / k) * t;
  temp = (d / t) * k;
  dT = t * (temp / k);
  n -= temp;
  dT += max(1, n / (2 * k)) * t;
  if (dT < sT)
    cout << "YES";
  else
    cout << "NO";
  cout << endl;
  return 0;
}
