#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int total = ((n + k - 1) / k) * t;
  if (total > t + d) {
    cout << "YES";
  } else {
    cout << "NO";
  }
}
