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