So first it would be 1 cm : 4 km, then you would have to convert it and I recommend you use Kilo Hecto Deca Unit Deki Centi Mili (or as I call it King Henry Drank Ucky Dark Chocolate Milk) so it would be 1 cm : 400,000 cm, because in order to simplify a ratio you need to first get it to the same unit. Anyways, there you go. It might be able to simplify more but I think that's the best you can do
