Let's consider addition of the first equiv of #H^(+)#, from the hydrogen bromide electrophile, and see what develops:
#"H"_2"C=CHCH=CH"_2+"HBr"rarr"H"_3"C-C"^(+)"HCH=CH"_2 +"Br"^-#
The proton adds to the olefin at #C_1#. Of course it could add at #C_2#, but here this would leave a primary carbocation rather than a secondary carbocation.
Moreover, addition at #C_1# leaves a carbocation intermediate that is resonance stabilized:
#H_3C-C^(+)H-CH=CH_2harrH_3C-CH=CH-C^(+)H_2#
Now, the bromide ion (delivered from the hydrohalide electrophile can potentially add at #C_1# or #C_4# on the hydrocarbon chain. We would expect both substitutions to occur, and it seems they do.
#"H"_3"CC(Br)HCH=CH"_2and"H"_3"CCH=CHCH"_2"Br"#
