The product formed has two chirality centres, therefore, four stereoisomers $\left(2^{2}\right)$ will exist.

$\begin{array}{*{20}{c}}
  {\,\,\,\,\,C{H_3}} \\ 
  | \\ 
  {C{H_3} - C{H_2} - C = CH - C{H_3}} 
\end{array}$ $\xrightarrow[{Peroxide}]{{HBr}}$ $\mathop {\begin{array}{*{20}{c}}
  {\,\begin{array}{*{20}{c}}
  {\,\,\,\,\,\,C{H_3}} \\ 
  | 
\end{array}} \\ 
  {C{H_3} - C{H_2} - {}_*CH - \mathop {CH}\limits^{*\,}  - C{H_3}} \\ 
  {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|} \\ 
  {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Br} 
\end{array}}\limits_{Major\,\,product} $