A block of mass 40 kg slides over a surface, when a mass of 4 kg is suspended through an inextensible massless string passing over frictionless pulley as shown below. The coefficient of kinetic friction between the surface and block is 0.02. The acceleration of block is: (Given $g = 10 \mathrm{~m~s}^{-2}$.)
(1) $\frac{8}{11} \mathrm{~m~s}^{-2}$
(2) $1 \mathrm{~m~s}^{-2}$
(3) $\frac{1}{5} \mathrm{~m~s}^{-2}$
(4) $\frac{4}{5} \mathrm{~m~s}^{-2}$