#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int t1 = (n - 1) / k * t, t2 = d + t;
  int case1 = max(t1, t2);
  int case2 = (n + k - 1) / k * t;
  cout << (case2 <= case1 ? "NO" : "YES");
}
