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