A tetrapeptide has –COOH group on alanine. This produce glycine (Gly) , Valine( Val) , phenyl alanine (phe) and alanine (Ala) on complete hydrolysis. For this tetra peptide, the number of possible sequences (primary structures) with  $-N{H}_2$  group attached to a chiral centre is 

Since –COOH group is attached to alanine, one terminal of the tetrapeptide contains alanine. Thus its position is fixed 
glycine-Achiral
$\begin{array}{l}valine\\ phenylalanine\end{array}\}$ chiral. Thus  $-N{H}_2$  group attached to either valine (or) phenyl alanine
Possible combinations
$H_{2}N$-val-Gly-phe-ala-COOH
$H_{2}N$-val-phe-Gly-Ala-COOH
$H_{2}N$-phe-Gly-val-Ala-COOH
$H_{2}N$-phe-val-Gly-Ala-COOH