Thank you for your interest in Flash Builder. The Free RIA Tools offer of complimentary licenses of Flash Builder for Students and Educators has expired.
Flash Builder 4.7 Premium is still available as a free trial either as a standalone product or as part of a free Creative Cloud subscription. In addition, Adobe is currently offering a significant discount on Creative Cloud subscriptions for Students and Educators.
Flash Builder standalone: https://www.adobe.com/cfusion/tdrc/index.cfm?product=flash_builder
Creative Cloud subscriptions for Students and Educators: https://creative.adobe.com/join/edu