#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, K, T, D;
  cin >> N >> T >> K >> D;
  int m = D + T;
  int r = (m / T) * K;
  if (r >= N)
    cout << "NO" << endl;
  else
    cout << "YES" << endl;
  return 0;
}
