Article: Strasbourg is situated on the eastern border of France with Germany. This border is formed by the River Rhine, which also forms the eastern border of the modern city, facing across the river to the German town Kehl. The historic core of Strasbourg however lies on the Grande Île in the River Ill, which here flows parallel to, and roughly 4 kilometres (2.5 mi) from, the Rhine. The natural courses of the two rivers eventually join some distance downstream of Strasbourg, although several artificial waterways now connect them within the city.

Question: The River Ill flows parallel with what river for 4 kilometres?
Rhine