#include <bits/stdc++.h>
using namespace std;
const long long _N = 5e5 + 11;
const long long INF = 1e18;
const long long MOD = 1e9 + 7;
const double EPS = 1e-7;
long long n, t, k, d;
long long cake(long long t1) {
  long long x = t1 / t;
  return x * k;
}
signed main() {
  ios_base ::sync_with_stdio(0);
  cin.tie();
  cout.tie();
  cout << fixed;
  cout << setprecision(15);
  srand(time(NULL));
  cin >> n >> t >> k >> d;
  long long x = n / k;
  if (x * k < n) x += 1;
  long long res1 = x * t;
  long long lo = d, hi = 2e9 + 1LL;
  while (lo + 1LL < hi) {
    long long md = (lo + hi) / 2LL;
    long long cake1 = cake(md);
    long long cake2 = cake(md - d);
    if (cake1 + cake2 >= n)
      hi = md;
    else
      lo = md;
  }
  if (hi < res1)
    cout << "YES" << '\n';
  else
    cout << "NO" << '\n';
  return 0;
}
