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

Here's why:

* Peptide Bond Formation: Each time an amino acid joins with another, a molecule of water is released. This process is called a dehydration reaction.

* Four Amino Acids: Since you have four amino acids, you need three peptide bonds to link them together.

* Total Water Molecules: This means three water molecules are formed in total.