3D equals three-dimensional. This refers to the added dimension of depth. Most modern movies have a 3D version, most of which require you to wear 3D glasses so that your eyes put the very odd-seeming pictures into an optical illusion that makes the image 'pop' out at you. How to Train Your Dragon and Cars 2 both have 3D versions for example. The version you watch at home is 2D unless you explicitly have the 3D version and a 3D capable TV.

4D technically refers to the fourth dimension, which is time. There is no such thing as a 4D movie.

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3D ultrasound, as the name suggests, gives a three-dimensional image. However, as is the convention, 3D describes a static image. With this technological development, you can see the baby in 3D and see the motion as well. It is the motion that is described as the fourth dimension hence the term 4D.
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