Sonography also known as USG or Sonogram is an imaging test offered during pregnancy for viewing developing foetus.
It uses sound waves to create images of structures within and so is very safe in pregnanacy.
It does not use radiation.
3D/4D as name suggests,probvides you. 3D/4D images of the baby. It is mostly used for better understanding of any problems/anomalies if detected on 2D USG.