A body slipping on a rough horizontal plane experiences a...

When a body slips on a rough horizontal plane, the only horizontal force acting on it is the kinetic friction.

The frictional force is given by $f_k = \mu_k N$, where $N = mg$ is the normal reaction.

The deceleration $a$ is given by Newton's second law:

$ma = \mu_k mg$

$a = \mu_k g$

Given $a = 4.0 \text{ m/s}^2$ and taking $g = 10 \text{ m/s}^2$:

$\mu_k = \dfrac{a}{g} = \dfrac{4.0}{10} = 0.4$