#include <bits/stdc++.h>
using namespace std;
int main() {
  std::ios::sync_with_stdio(0);
  cin.tie(0);
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int t1, t2;
  t1 = ((long long int)(ceil)((float)n / (float)k)) * t;
  t2 = d + t;
  if (t2 < t1)
    cout << "YES";
  else
    cout << "NO";
}
