Where does the 4000 figure come from?
The rig veda is probably what most people agree is one of the foundational texts of hinduism, but it dates to about 1500 BC.
However, the vedas probably absorbed a lot of earlier cultural tradition from the indus valley civilization, especially worship of Shiva-like deities. How much is however, debated.
just like mesopotamia, there was a lot of syncretism.