When potassium iodide is added to an aqueous solution of potassium ferricyanide, a reversible reaction is observed in which a complex $\mathbf{P}$ is formed. In a strong acidic medium, the equilibrium shifts completely towards $\mathbf{P}$. Addition of zinc chloride to $\mathbf{P}$ in a slightly acidic medium results in a sparingly soluble complex Q.

The number of zinc ions present in the molecular formula of $\mathbf{Q}$ is _______.