What is 4D ultrasound scan?

What is 4D scan or 4D ultrasound? What is the difference between 3D and 4D? Educational video clip about medical ultrasound technology: 2D - is real-time and uni-planar image of the fetus 3D - is still volume, and it is created from multiple 2D pictures of the baby 4D - is moving in real-time 3D image of the fetus