4D anyone?

Kit • Army Ranger wife 3/31/14, pharm student, 1st time mommy to be 8/8/15 🎀💞
My husband and I an appt today to do a 4D ultrasound on our baby and I was curious to know has any other mommies out there done one. What's the difference between 3D and 4D. I'm 18wks and 3daysvdo you think they would be able to see who the baby resembles?