Understand from my Gynae that performing a 4D scan is just a Novelty and it is not worth paying $300+ for it. It is only required should you want to check if baby has cleft lip. As long as the Gynae does not detect any defects in the earlier tests and ultra scans in your first and second trimester, a 4D Scan is not necessary.
I have chosen to forgo the scan too. The Gynae jokingly say that whatever baby look like in the 4D scan, he/she will look totally different at birth. It is better to spend the money on a professional baby shoot when the baby is born instead.