4000 years ago is 2000 BC
The horse drawn chariot was just coming into use
Polygamy allowed one man to create enormous amount of
descendants in just few generations
Chariot borne warriors to advance much quicker and farther than earlier migrations
Corresponds with the mythical coming of the “ARYANS”
Supposedly much of Central Asia is descended from Gengis Khan
The Bronze Age King invented the 4 wheeled horse drawn chariot with a back seat which was, BTW, the only one in Europe, known as the Mercedes-Bender, thus allowing make-outs at every drive-in Stone Henge. This increased the population overnight, as it were, and his horses were very very fast.
It wasn’t just polygamy that helped, but concubinage. Kings of Siam, Turkey and others had hundreds of concubines and wives. I’m not sure that thy coming of the “Aryans”is entirely mythical. Something major happened to Europe around that time and it came from the East.