#include <bits/stdc++.h>
int main() {
  int n, k, d, t;
  while (scanf("%d %d %d %d", &n, &t, &k, &d) != EOF) {
    int atime;
    if (n % k == 0)
      atime = n / k;
    else
      atime = n / k + 1;
    int flag = 1;
    if (atime == 1) flag = 0;
    if (atime > 1 && d >= (atime - 1) * t) flag = 0;
    if (flag)
      printf("YES\n");
    else
      printf("NO\n");
  }
  return 0;
}
