Apple has just released the fourth iOS 6.1 beta to developers. The release has a build number of 10B5126b and is available on Apple’s Developer page or via OTA update.
Apple has also released beta 3 for Apple TV and Xcode 4.6 Developer Preview 4. All of these betas can be downloaded from Apple’s Developer page. Beta 4 for iOS 6.1 is available via OTA update for those who already have a beta version of iOS 6.1 installed.