No. of possible sequences (primary structures) with $-\text{N}{\text{H}}_2$ group attached to a chiral center is 4.
Since $\text{-COOH}$ group is attached to alanine, one terminal of the tetrapeptide contains alanine. Thus, its position is fixed.
Glycine is achiral whereas valine and phenylalanine are chiral. Thus, ${\text{-NH}}_{\text{2}}$
group is attached to either valine or phenylalanine.
Thus following are the four possible combinations of the tetrapeptide.
${\text{H}}_{\text{2}}\text{}\text{N}-\text{Val}-\text{Gly}-\text{Phe}-\text{Ala}-\text{COOH}$
${\text{H}}_{\text{2}}\text{}\text{N}-\text{Val}-\text{Phe}-\text{Gly}-\text{Ala}-\text{COOH}$
${\text{H}}_{\text{2}}\text{}\text{N}-\text{Phe}-\text{Gly}-\text{Val}-\text{Ala}-\text{COOH}$
${\text{H}}_{\text{2}}\text{}\text{N}-\text{Phe}-\text{Val}-\text{Gly}-\text{Ala}-\text{COOH}$