The hydrogen bromide addition in peroxides follows Anti Markovnikov rule.  Anti Markovnikov  addition reaction is found to follow a free radical mechanism. The peroxide compound involved helps in the generation of free radicals. The Anti-Markovnikov rule defines regiochemistry in which the substituent is attached to a less substituted carbon instead of the more substituted carbon.

The reaction of $3-$Methylhex-2-ene  with $\mathrm{HBr}$ in presence of peroxide, takes place as follows,

$2$ chiral carbons, here, so, total number of stereoisomers will be $2^{\mathrm{n}}=2^2=4$