For 3D ultrasounds, multiple two-dimensional images are taken at various angles and then pieced together to form a three-dimensional rendering. For instance, instead of just seeing a profile view of your cutie’s face, in a 3D sonogram you can see the whole surface (it looks more like a regular photo).

A 4D ultrasound is similar to a 3D ultrasound, but the image shows movement. That means like a video, in a 4D sonogram you see your baby doing things in real time (like opening and closing his eyes and sucking his thumb).

Source
https://www.whattoexpect.com/pregnancy/pregnancy-health/prenatal-testing-3d-4d-ultrasounds/