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