The effective resistance of several resistors in series
is the sum of their individual resistances.
In this circuit, the effective resistance is
(2 ohms + 2 ohms + 2 ohms + 6 ohms) = 12 Ω .
So 12 ohms is what the battery sees. The battery doesn't know
whether that's a single resistor, or the sum of several.
Now, the voltage across a resistor is
V = (current) · (resistance) .
For this circuit ...
V = (4.0 A) (12 Ω) = 48 volts .