To write "5 is to 1 as 20 is to 4" in fractional notation, you would express it as:
\[ \frac{5}{1} = \frac{20}{4} \]
So the correct option is:
\[ \frac{5}{1} = \frac{20}{4} \]
This can be rewritten as:
5 : 1 :: 20 : 4
The correct choice from the given options is:
5 : 1 = 20 : 4