Apple is now 4 for 4.
Or at least Beta 4 for iPhone OS 4.0.
They have just posted the latest and greatest iPhone OS 4.0 beta. As with previous beta releases, this new beta update provides firmware for each of the supported devices (namely the iPhone 3GS, the iPhone 3G, and the 2nd and 3rd generation iPod touch).
Because of the ongoing NDA, the details of this release remain behind a wall of confidentiality. You'll need to visit the developer site and check out the specifications for yourself. Apple will be releasing 4.0 into the general iPhone population this summer. An iPad version will debut in the fall.
Apple's servers get slammed whenever a new SDK is released. You may want to wait a few hours if you encounter any problems connecting to the developer site.