[MRMEAN]

They counted. See Article Nature 437, 69-87 (1 September 2005) | doi: 10.1038/nature04072
Initial sequence of the chimpanzee genome and comparison with the human genome
The Chimpanzee Sequencing and Analysis Consortium25 which is one of the article's links.

[MEGoody]

Ah, so what they really mean is that there are 4 million differences between chimp and human DNA, not that there were 4 million evolutionary events.