Which of the following is not an allylic halide?

An allylic halide is a compound in which the halogen atom is bonded to an $sp^3$ hybridized carbon atom that is adjacent to a carbon-carbon double bond.

Let us examine the structure of each option:

4-bromopent-2-ene: $CH_3-CH=CH-CH(Br)-CH_3$
The bromine atom is attached to C4, which is an $sp^3$ carbon adjacent to the C2=C3 double bond. It is an allylic halide.

3-bromo-2-methylbut-1-ene: $CH_2=C(CH_3)-CH(Br)-CH_3$
The bromine atom is attached to C3, which is an $sp^3$ carbon adjacent to the C1=C2 double bond. It is an allylic halide.

1-bromobut-2-ene: $Br-CH_2-CH=CH-CH_3$
The bromine atom is attached to C1, which is an $sp^3$ carbon adjacent to the C2=C3 double bond. It is an allylic halide.

4-bromobut-1-ene: $CH_2=CH-CH_2-CH_2-Br$
The bromine atom is attached to C4, which is an $sp^3$ carbon adjacent to C3 (another $sp^3$ carbon), not the double bond. Thus, it is not an allylic halide.

Answer: 4-bromobut-1-ene