In the arrangement shown in the figure, the coefficient of friction between two blocks is 0.5. The force of friction between the two blocks is (Assume that the 4 kg block is placed on a smooth horizontal surface)

(Acceleration due to gravity = 10ms^-2)

The FBDs of the blocks will be 

If we assume the blocks move with same acceleration,

${\mathrm{a}}_{\mathrm{c}}=\frac{\mathrm{f}-2}{2}=\frac{20-\mathrm{f}}{4}$

$2 \mathrm{f}-4=20-\mathrm{f}$

$\mathrm{f}=8 \mathrm{N}$

Maximum friction between the two blocks can be

${\mathrm{f}}_{\max }=\mu {\mathrm{m}}_{\mathrm{A}}\mathrm{g}=(0.5)\left(2\right)\left(10\right)=10 \mathrm{N}$

So, the blocks will move together where the friction between the blocks will be 8N.