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