#include <bits/stdc++.h>
using namespace std;
using namespace std;
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
int main() {
  int n, t, d, k, old, tmpo = 0;
  cin >> n >> t >> k >> d;
  old = ((n + k - 1) / k) * t;
  int ptr = 0;
  while (ptr < d && tmpo < n) {
    if (ptr + t > d) break;
    tmpo += k;
    ptr += t;
  }
  if (tmpo >= n)
    cout << "NO\n";
  else {
    int c = d % t;
    ptr = d + 1;
    while (true) {
      if (ptr % t == 0) tmpo += k;
      if (ptr % t == c) tmpo += k;
      if (tmpo >= n) break;
      ptr++;
    }
    if (old <= ptr) {
      cout << "NO\n";
    } else {
      cout << "YES\n";
    }
  }
  return 0;
}
