Since the resistances $R_{2}$ and $R_{3}$ are in parallel, their equivalent resistance will be $R_{23}=\frac{4 \times 4}{2 \times 4}=2 \,\Omega$
Now the equivalent circuit will be
There are three resistances in series.
Therefore the equivalent resistance will be $R_{ AB }=2+2+2=6 \,\Omega$