From Mechanics

As the block is moving relative to the surface the friction force must satisfy

\displaystyle F = \mu R

Also as the block has conmstant velocity the forces on it must be in equilibrium.

Resolving horisontally,

\displaystyle \begin{align} & \to :\quad 20\ \text{N}-F=0 \\ & \\ & F=20\ \text{N} \\ \end{align}

Resolving vertically,

\displaystyle \uparrow :\quad R-mg=0

\displaystyle R=mg=4\times 9\textrm{.}81=39\textrm{.}24\ \text{N}

Substituting in the friction equation,

\displaystyle 20 = \mu \times 39\textrm{.}24

giving

\displaystyle \mu = 0\textrm{.}51