Which of the following alkenes give a primary alkyl bromide as the MAJOR product with $\mathrm{HBr}$ in the presence of a peroxide?

In the radical path $\mathrm{Br}{}^{\bullet }$ adds to the less hindered, less substituted carbon of the double bond so as to give the more stable radical. Propene gives 1-bromopropane, 2-methylpropene gives 1-bromo-2-methylpropane, and styrene gives (2-bromoethyl)benzene. But-2-ene is symmetric and gives 2-bromobutane, a secondary bromide, either way. Answer: 1, 2 and 4.