A lot of people think that 4D ultrasound is a clearer form of 3D Ultrasound or ask if they're going to get "4D pictures." They misunderstand what 4D ultrasound means. 4D ultrasound is the ability to see the baby move in real time 3D. The live motion is the primary difference between 3D and 4D ultrasound.
When 3D ultrasound first came about the machine was only able to save a single image. When the ability to take real time video inside the womb came about the marketing term for it was 4D, being since the dimension of time was added.
Another very cool aspect of most 3D/4D ultrasound machines is that they are able to rotate the image of the unborn baby to show alternate angles and images.
These days just about any place that advertises 3D ultrasound also provides 4D ultrasound, provided they give you your video. If you are considering a 3D ultrasound, just ask what's included and be sure the place includes both pictures and video in your 3D/4D ultrasound package.