#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int n, t, k, d;
  scanf("%d %d %d %d", &n, &t, &k, &d);
  n -= ((d + t - 1) / t) * k;
  if (d % t == 0) n -= k;
  printf(n > 0 ? "YES\n" : "NO\n");
  return 0;
}
