#include <bits/stdc++.h>
using namespace std;
int main() {
  int h, t, k, d;
  cin >> h >> t >> k >> d;
  int t1 = t, k1 = k;
  for (;;) {
    if (k1 < h) {
      k1 += k;
      t1 += t;
    } else
      break;
  }
  if (t1 - (d + t) > 0)
    cout << "YES";
  else
    cout << "NO";
}
