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: $\mathrm{Tuftsin} \left\{(\mathrm{L}-\mathrm{threonyl})-(\mathrm{L}-\mathrm{lysyl})-(\mathrm{L}-\mathrm{prolyl})-(\mathrm{L}-\mathrm{arginine})\right\}$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) $=1$