In the $\mathrm{ccp}$ lattice, the number of octahedral voids are four and the number of tetrahedral voids are eight.

The number of atoms of $\mathrm{B}= 4$

The number of atoms of $\mathrm{A} =\frac{2}{3}\times 8=\frac{16}{3}$

So,

 $$\begin{gathered} \mathrm{A}:\mathrm{B}=\frac{16}{3}:4 \\ \mathrm{A}:\mathrm{B}=16:12 \\ \mathrm{A}:\mathrm{B}=4:3 \end{gathered}$$

Hence, the molecular formula of the compound is ${\mathrm{A}}_4{\mathrm{B}}_3$.