The total amount is 12 ounces therefore you can use that as your denominator.
4 ounces out of it are vanilla and the other 8 are chocolate.
4 out of 12 is vanilla . This can also be written as 4/12.
4/12 can be your first fraction.
The second one can be a simplified version of 4/12.
The simplified version would be 1/3.
Your two fractions would be 4/12 and 1/3.
Hope This Helps You!
Good Luck Studying :)