The four oldest civilizations are Mesopotamia, Egypt, the Indus valley, and China as they provided the basis for continuous cultural development in the same geographic location.
For further reading check the following articles:
|Major Sites in Indus Valley Civilization||Indus Valley Civilization|
|100 Must-Know Facts about IVC for UPSC||UPSC FAQ|
|NCERT Notes: Arts of the Indus Valley||NCERT notes for History (Modern, Medieval, Ancient), Art and Culture, and Geography|