Apple has released the fourth version of its iPhone Software Development Kit (SDK) .

A note on 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."

Open GL ES support is the latest addition to the software, which Apple last night explained had been downloaded 200,000 times, suggesting huge interest in developing applications for the platform.

The SDK also offers a much improved Audio Toolbox, NSXML Parser support and the UIFont has been redesigned. Code-signing is now enforced within the beta development platform.

Apple confirmed strong interest from among Fortune 500 companies, suggestive of significant potential adoption by enterprise users. Over a third of companies in the Fortune 500, and over 400 higher education institutions have applied to become iPhone developers.

Apple is working very hard to roll out the iPhone to more places in Europe and Asia during the year.