UPDATE: Zune 4.0 Available for Download.

Over a year since it updated the Zune software and the line-up of digital media players, Microsoft is gearing up to move to the next stage in the evolution of its iPod alternative. Zune 4.0 is currently scheduled for availability on September 15th, 2009. If the date sounds familiar to Zune owners, or future customers, it’s because it is synonymous with the deadline set by the Redmond-based company for the availability of its next generation of Zune devices. In mid-August 2009, the software giant revealed that Zune HD would be made available for purchase come September 15th.

The software giant hasn’t exactly officially revealed that it will deliver the Zune software refresh on September 15th. The date comes from a newsletter sent to Zune customers, designed to give a heads up as to the imminent availability of the update. “Soon you'll be able to experience the all-new Zune software packed with exciting new features. Watch for the update on September 15 so you'll be ready to enjoy the full Zune experience,” reads an excerpt out of the newsletter for Zune via engadget.

Microsoft has traditionally upgraded both the Zune software and its line-up of digital media players on a yearly basis since the introduction of Zune 1.0. Additionally, the software giant has always unveiled its new devices and software in the fall of each year. Zune 3.0, for example, was launched in the first half of September 2008.

As of just 11 days from now, customers will be able to acquire Zune HD, Microsoft’s response to iPod touch. Featuring an OLED touch screen and natural user interface (NUI) capabilities, along with built-in HD Radio receiver, HD video output capabilities, Wi-Fi, and powered by the NVIDIA Tegra ultralow-power HD processor, Zune HD has been available for pre-order as of August 2009. Customers have been able to buy Zune HD 16GB for $219.99 and Zune HD 32GB for $289.99, but additional offerings will be made available on September 15th. Recent rumors also pointed out that Microsoft has discontinued all older Zune devices, and that moving forward will focus entirely on Zune HD.