#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int count = 0;
  int sum = 0;
  while (sum < n) {
    count += t;
    sum += k;
  }
  int count2 = d;
  int sum2 = (d / t) * k;
  while (sum2 < n) {
    count2 += t;
    sum2 += (2 * k);
  }
  if (count2 < count) {
    cout << "YES" << endl;
  } else {
    cout << "NO" << endl;
  }
}
