The Diagram shown in the question represents the balanced wheatstone bridge. It satisfy the condition of balanced wheat stone bridge.
Multiplication of dimensionally opposite resistance is same.
$2\times 2=2\times 2\Rightarrow 4=4$
When the circuit is balanced wheat stone bridge then current through the branch $BD$ is zero, so it can be removed.

Now, after removing $BD$ wire voltage across $AC$ is $8 \mathrm{V}$.

So, current will be,

${\mathrm{i}}_2=\frac{8}{4+4}=1 A$