So how is 4D ultrasounds different than 3D ultrasounds? A very common question. The easiest way to describe 4D ultrasound is” video” or “movement”. 3D is just a still shot, but 4D is 3D “live” or 4th dimension (movement) or even 4D video. Put another way, it is the color images you see (3D ), but in movement (4D). Because it is “live” or in “movement,” this video is often saved as a video file in order to be able to playback in the future.