Apple has released the fourth beta version of its iPhone software development kit.

Apple's developer website explains: "The fourth beta version of the iPhone SDK includes Xcode IDE, iPhone simulator with Open GL ES support, Interface Builder, Instruments, frameworks and samples, compilers, and Shark analysis tool". The SDK also offers an improved Audio Toolbox, NSXML Parser support and redesigned UIFont.

This new release comes as Apple executives discussed the SDK during its second quarter finanical results press conference call.

The company said they have had "strong interest" from among Fortune 500 companies, with over a third of companies in the Fortune 500, and over 400 higher education institutions applying to become iPhone developers. This bodes well for enterprise level apps for the phone.

Apple CFO Peter Oppenheimer revealed that the company has signed up 200,000 developers with more signing up each week and that the response to the SDK was "tremendous".