#include <bits/stdc++.h>
using namespace std;
long long modpow(long long x, long long n, long long m) {
  if (n == 0) return 1 % m;
  long long u = modpow(x, n / 2, m);
  u = (u * u) % m;
  if (n % 2 == 1) u = (u * x) % m;
  return u;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int c1 = 0, ans1 = 0;
  int c2 = 0, ans2 = 0;
  for (int i = 0; i <= 1e9; ++i) {
    if (i > 0 && i % t == 0) c1 += k;
    if (c1 >= n) {
      ans1 = i;
      break;
    }
  }
  for (int i = 0; i <= 1e9; ++i) {
    if (i > 0 && (i % t == 0)) c2 += k;
    if (i >= (d + t) && ((i - d) % t == 0)) c2 += k;
    if (c2 >= n) {
      ans2 = i;
      break;
    }
  }
  if (ans2 >= ans1) {
    cout << "NO";
  } else
    cout << "YES";
  return 0;
}
