In a linear tetrapeptide (Constituted with different amino acids),...

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