#include <bits/stdc++.h>
int main() {
  int n, t, k, d, a, i, b;
  scanf("%d %d %d %d", &n, &t, &k, &d);
  a = ceil(1.0 * n / k) * t;
  for (i = 0; (i + 1) * t <= d; i++)
    ;
  b = d + ceil(1.0 * (n - (i * k)) / (2 * k)) * t;
  if (b < a)
    printf("YES");
  else
    printf("NO");
}
