#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long n, t, k, d;
  cin >> n >> t >> k >> d;
  long long T1 = ((n + k - 1) / k) * t;
  if (max(d + t, T1 - t) < T1) {
    puts("YES");
  } else
    puts("NO");
}
void testgen() {
  FILE* f = fopen("input.txt", "w");
  srand(time(0));
  fclose(f);
}
int main(int argc, char* argv[]) {
  cout << fixed;
  cout.precision(4);
  cerr << fixed;
  cerr.precision(12);
  solve();
  return 0;
}
