#include <bits/stdc++.h>
using namespace std;
const int INF = 1 << 29;
inline int in() {
  int x;
  cin >> x;
  return x;
}
template <class T>
void max_self(T &a, const T &b) {
  if (a < b) a = b;
}
template <class T>
void min_self(T &a, const T &b) {
  if (a > b) a = b;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int N, time, cap, build;
  cin >> N >> time >> cap >> build;
  int totalT = ((N / cap) + (N % cap > 0)) * time;
  if (build + time < totalT)
    cout << "YES"
         << "\n";
  else
    cout << "NO"
         << "\n";
  return 0;
}
