Here's why:

* Hydrogen Bonding: Hydrogen bonds are a type of intermolecular force that occurs between a hydrogen atom covalently linked to a highly electronegative atom (like nitrogen, oxygen, or fluorine) and a lone pair of electrons on another electronegative atom.

* Ammonia Structure: Ammonia has a central nitrogen atom with three hydrogen atoms bonded to it. The nitrogen atom also has one lone pair of electrons.

* Hydrogen Bond Formation: The lone pair on the nitrogen can form a hydrogen bond with a hydrogen atom of another ammonia molecule. Additionally, each of the three hydrogen atoms on ammonia can form a hydrogen bond with the lone pair on the nitrogen of another ammonia molecule.

Therefore, one ammonia molecule can form one hydrogen bond with its lone pair and three hydrogen bonds with its three hydrogen atoms, making a total of four hydrogen bonds.