The formula with 4 substituted
US gallons = 4 ÷ 3.785411784 = 1.057
The constant is an exact definition, not a rounded factor: the mile is 1.609344 km, the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres. The Litres to Gallons Converter explains where each comes from and gives a longer table.
4 litres is 1.057 US gallons and 0.880 imperial gallons. The imperial gallon is 4.54609 litres against the US gallon's 3.785411784, which is why the two answers differ by a fifth.
What 4 litres means in practice
4 litres is 1.057 US gallons, just over one, and 0.880 imperial gallons. A 4.0 L engine is a large one by today's standards; a 4 litre fuel can is the small size sold at most petrol stations.