Each branch in a parallel circuit has the same voltage across it, so the voltage in the top branch IS 18V.
Now, you need to know the current in the top resistors.
Rtop branch= 4 + 8 = 12 ohms
Thus
Itop = V/Rtop = 18/12 = 1.5A
SO, the voltage across the 4ohm resistor is
V4ohm = IR = 1.5 x...