A block of mass $M$ placed inside a box descends vertically with acceleration $a$. The block exerts a force equal to one-fourth of its weight on the floor of the box. The value of $|a|$ will be\\
(1) $g$\\
(2) $\frac{3g}{4}$\\
(3) $\frac{g}{2}$\\
(4) $\frac{g}{4}$