Hi Nishu... Looks like you haven't understood the concept of ratio and proportion. Here follow the following steps -
(1) Water : Milk for 1 Cup = 3 : 1
(2) We must write the ratio as a fraction = 3/1
(3) Now, we multiply 4 with both numerator and denominator = 12/4
(4) So, the ratio of Water :Milk for 4 cups of Tea is 12 : 4.
Hope I could help you understand.... Looking forward to help you again;))