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