#include <bits/stdc++.h>
using namespace std;
const int INF = 1 << 29;
const int MOD = 1000000007;
bool isPowerOfTwo(long long int x) { return x && (!(x & (x - 1))); }
void fastio() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
}
long long binpow(long long a, long long b) {
  long long res = 1;
  while (b > 0) {
    if (b & 1) res = res * a;
    a = a * a;
    b >>= 1;
  }
  return res;
}
const int dx[] = {1, 0, -1, 0, 1, 1, -1, -1};
const int dy[] = {0, -1, 0, 1, 1, -1, -1, 1};
string solve() {
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int time = n / k * t + (n % k ? t : 0);
  if (d < time - t) return "YES";
  return "NO";
}
int main() {
  fastio();
  int t = 1;
  while (t--) {
    cout << solve();
  }
  return 0;
}
