The equivalent resistance of the circuit shown below between points a and b is:

Due to the symmetric bridge structure between points a and b, the central vertical $$4\ \Omega$$ resistor carries no current and is removed from the network.

This leaves three parallel branches connected across terminals a and b:

Top branch: Two $$4\ \Omega$$ resistors in series = $$4 + 4 = 8\ \Omega$$

Middle branch: Two $$4\ \Omega$$ resistors in series = $$4 + 4 = 8\ \Omega$$

Bottom branch: One $$16\ \Omega$$ resistor

$$R_{\text{parallel}} = \frac{8 \times 8}{8 + 8} = 4\ \Omega$$

$$R_{ab} = \frac{4 \times 16}{4 + 16} = \frac{64}{20} = 3.2\ \Omega$$