In the arrangement shown in figure. The strings are light and inextensible. The surface over which blocks are placed is smooth.  the tension in the  string  between 4kg and 2 kg blocks is 

 Free body diagram (showing the forces in x-direction only) of 4kg block and 1kg block are shown in Fig. 

$$\begin{gathered} \text{ Using } \Sigma F_x =m{a}_x \\ \text{ For l'kg block, } F-T_1 =\left(1\right)\left(a\right) \\ \text{ or } 14-T_1 =\left(1\right)\left(2\right)=2 \\ \therefore T_1 =14-2=12 \mathrm{N} \\ \text{ For }4 \mathrm{kg}\text{ block, } T_2 =\left(4\right)\left(a\right) \\ \therefore T_2 =\left(4\right)\left(2\right)=8 \mathrm{N} \end{gathered}$$