The number of stereoisomers possible for a compound of the molecular formula ${\text{CH}}_3-\text{CH}=\text{CH}-\text{CH}(\text{OH})\text{Me}$ is:

The molecule has two centers of stereoisomerism:One double bond (${\text{CH}}_3-\text{CH}=\text{CH}-$) which can exist in $cis$ and $trans$ (or $E$ and $Z$) configurations.One chiral carbon atom ($\text{CH}(\text{OH})\text{Me}$) which can exist in $R$ and $S$ configurations.Since the molecule is unsymmetrical, the total number of stereoisomers is $2^n$, where $n$ is the number of stereocenters.
$$n=2⟹\text{Total isomers}=2^2=4$$
The four isomers are: ($cis,R$), ($cis,S$), ($trans,R$), and ($trans,S$).