If the new fraction is 8/12 I will show how to find it.
Let's say the original denominator is x, then the numerator is x-4, so we have (x-4)/x.
Then it says f we add 4 to both, so now we have x/(x+4) this equals 8/12. Now, keep in mind -4 cannot be an answer because it would make x/(x+4) be divided by 0 which wouldn't exist.