#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int tmp1 = (n + k - 1) / k * t;
  int tmp2 = d;
  for (int j = tmp2 + 1; j < tmp1; ++j) {
    int done1 = j / t * k;
    int done2 = (j - tmp2) / t * k;
    if (done1 + done2 >= n) {
      puts("YES");
      return 0;
    }
  }
  puts("NO");
}
