#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, t, d, p = 0, o = 0;
  cin >> n >> k >> t >> d;
  if (t >= n) {
    cout << "NO" << endl;
    return 0;
  } else {
    if (n % t == 0) {
      p = ((n / t) * k);
    } else {
      p = (((n / t) + 1) * k);
    }
    o = d + k;
    if (o >= p)
      cout << "NO" << endl;
    else
      cout << "YES" << endl;
  }
  return 0;
}
