#include <bits/stdc++.h>
using namespace std;
int main() {
  double n, t, k, d;
  cin >> n >> t >> k >> d;
  double totalTime = ceil(n / k) * t;
  double totalTime2 = d;
  double time = ceil(d / t);
  if (t <= d) n -= (k * time);
  if (n <= 0)
    cout << "NO";
  else {
    totalTime2 += (ceil(n / (k * 2)) * t);
    if (totalTime <= totalTime2)
      cout << "NO";
    else
      cout << "YES";
  }
  return 0;
}
