The Nile River is a major river in Africa, and is often referred to as the 'Blue River' due to its color. It is the longest river in the world, stretching over 4, 000 miles from its source in the Ethiopian Highlands to its mouth in the Mediterranean Sea. The Nile is an important source of water for many countries in the region, and is also a major transportation route. The four-letter answer to the crossword puzzle is NILE.