When four amino acids join together to form a polypeptide chain, three water molecules are formed.

Here's why:

* Dehydration Synthesis: The formation of a peptide bond between amino acids involves a dehydration reaction, where a water molecule is removed.

* Number of Bonds: Four amino acids require three peptide bonds to connect them.

* Water Molecules: Since each peptide bond releases one water molecule, three peptide bonds will result in three water molecules being formed.