A tetrapeptide is a peptide, classified as an oligopeptide, since it only consists of four amino acids joined by peptide bonds.
A peptide bond, also referred to as an amide bond, is formed between the α-nitrogen atom of one amino acid and the carbonyl carbon of a second.
Example of tetrapeptide is: Tuftsin(L−threonyl)−(L−lysyl)−(L−prolyl)−(L−arginine)is a peptide related primarily to the immune system function
As we can see in this example
No. of Amino Acids =4
No. of Peptide bonds =3
Hence, (number of amino acids) - (number of peptide bonds) =1