3d Ultrasound as the name sounds, generates images in 3 dimensions by adding depth to the image. By sending ultrasound waves in different directions, measuring the waves that bounce back and using some advanced mathematical Algorhythm, images with depth can be created. The primary application of 3d ultrasound is obstetrics, but 3d ultrasound can be to image many other parts as well.
4d Ultrasound adds the dimension of time to a 3d image. This means that instead of a still picture that is the result of a 3d ultrasound , you get a series of still images taken closely together. When these images are viewed rapidly in succession our eyes perceive movement. Put another way, a 4d ultrasound is a video of a 3d ultrasound.
There is a debate within the ultrasound community about whether or not 3d/4d ultrasound should be used for non-medical purposes. The official position of the FDA is that while there are no known risks, it does affect the body by tissue heating and cavitation which. Therefore the FDA discourages the use of using this technology for non-medical purposes, i.e. keepsake videos.