Apple has released the iPhone OS 4.0 beta 4 build 10M2252 for developers which definitely includes bug fixes and enhancements but right now they did not provide any change log for this version as compare to beta 3.
The registered developers at Apple iPhone Developer Program can grab the latest iPhone OS beta through their accounts. The iPhone OS 4 beta 4 works for iPhone 3GS, 3G, and iPod Touch 2G, 3G.
We will update this article as we find new features of iPhone OS 4.0 beta 4.
Download iPhone OS 4.0 Beta 4