#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, k, d, ans1, ans2;
  cin >> n >> t >> k >> d;
  if (n % k == 0)
    ans1 = (n / k) * t;
  else
    ans1 = (1 + n / k) * t;
  n = n - (d / t) * k;
  k *= 2;
  if (n % k == 0)
    ans2 = (n / k) * t + d;
  else
    ans2 = (1 + n / k) * t + d;
  if (ans2 < ans1)
    cout << "YES";
  else
    cout << "NO";
  return 0;
}
