#include <bits/stdc++.h>
using namespace std;
int main() {
  float num;
  int t, n, d;
  cin >> num >> t >> n >> d;
  int a = (num / n);
  if ((num / n) > a) a += 1;
  int y = d, x = 0;
  for (int i = 0; i < a; i++) {
    if (x <= y)
      x += t;
    else
      y += t;
  }
  if (max(x, y) < (t * a))
    cout << "YES";
  else
    cout << "NO";
  return 0;
}
