I found this article interesting:
http://www.livescience.com/25117-musicians-brains-sync-up-during-duet.html
The study at the Max Planck Institute in Berlin found that the brain waves of two musicians synchronize when the two play a duet. I imagine the same principle applies to four musicians playing a quartet.
We in the quartet often refer to the concept of the ‘mind meld,’ and here is the science to back up that idea.