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