Whereas 3D is an ultrasound technique used to render a structure in 3 dimensions, 4D ultrasound is the same thing but adding the dimension of time or motion.

Click on the links to see some cute (looped) videos of [Flash required]:

baby yawning - baby wriggling toes - baby lifting foot up - baby's itching eye - baby licking fingers - baby caressing face - baby rubbing eye with foot - baby smiling.