If the current flowing through 3 ohm resistor is 0.8 ampere in the given circuit, then what is the potential drop across the 4 ohm resistor?

4.8 V

$\text{If Current through 3}\Omega \text{ is }  I_1 \text{and Current through 6}\Omega \text{ is }  I_2$

$\text{then } I_2 = \frac{R_1}{R_2} I_1 = 0.4 A$

$\Rightarrow \text{Current through 4}\Omega = I_1 +I_2 = 1.2A$

$\Rightarrow V = IR = 4.8V$