To write the statement "5 is to 1 as 20 is to 4" as a proportion in fractional notation, we express it in the form of fractions.
The statement can be written as: \[ \frac{5}{1} = \frac{20}{4} \]
So, the answer would represent this proportion in fractional notation, which matches: 5:1 :: 20:4 (ratio of 5 to 1 colon colon ratio of 20 to 4).
This simplifies the answer and captures the essence of the question.