Understanding computation of voltage
If you use KVL to solve this, you'll be done...the result will include the voltage you need without any need to apply the current divider rule.

If you just want to solve the problem, notice that the voltage across the 4-ohm resistor is exactly the same as the voltage across the 2-ohm resistor. So if you take the equivalent resistance by combining these parallel resistors, and solve for the voltage across that equivalent resistor, you'll have the voltage you want.