#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  int a = 0, b = 0, c = 0, sum1 = 0, d = 0;
  cin >> a >> b >> c >> d;
  for (; sum1 <= d;) {
    sum1 += b;
    a -= c;
  }
  if (a > 0)
    cout << "YES";
  else
    cout << "NO";
}
