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