Four identical discs each of mass '$\mathrm{M}$' and diameter '$\mathrm{a}$' are arranged in a small plane as shown in figure. If the moment of inertia of the system about $\mathrm{OO}^{\prime}$ is $\frac{x}{4} \,\mathrm{Ma}^{2}$. Then, the value of $x$ will be ____________.