#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, k, d;
  scanf("%d%d%d%d", &n, &t, &k, &d);
  int t1 = d / t;
  if (d % t) t1++;
  int num = n - t1 * k;
  if (num > k)
    printf("YES\n");
  else if (num > 0 && (d % t))
    printf("YES\n");
  else
    printf("NO\n");
  return 0;
}
