#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int m1 = n / k;
  if (n % k != 0) m1++;
  m1 *= t;
  if (d + t < m1)
    printf("YES\n");
  else
    printf("NO\n");
}
