2,5-Dibromohex-3-ene, $\mathrm{CH}{}_3-\mathrm{CHBr}-\mathrm{CH}=\mathrm{CH}-\mathrm{CHBr}-\mathrm{CH}{}_3$, has two stereocentres and a C=C bond. Counting all configurational isomers, the number of them that are optically active (chiral) is

Both ends are identical, so for each geometry (E or Z) the possibilities are (R,R), (S,S) and (R,S)=(S,R) meso: 3 per geometry and 6 stereoisomers in all. The meso forms (one E, one Z) are achiral, and the (R,R)/(S,S) forms of each geometry are chiral. Optically active = 2 x 2 = 4.