What is the difference between 4D and 3D FOR ULTRASOUND?

3D Ultrasound shows 3D still images of the baby ,in other hand the 4D Ultrasound shows a video of the baby in 3D . You can say 4D Ultrasound is a 3D images added the time factor.

3D scans show still pictures of your baby in three dimensions. 4D scans show moving 3D images of your baby, with time being the fourth dimension. 

The 3D shows a still image while the 4D shows moving 3D image.

The 3D allows aus to have static images of internal anatomy

The 4D allows us to add live streaming video of the images

3D Scan Show still Image of Baby while 4D show the moving 3D image with respect to time. In other words 4D ultrasound gives the three dimensions (x,y,z) with time span.  

3D means three dimension which shows only pictures

4D means  four dimension which shows moving .

4D is change in time, so you have a volume presented also.