Flash Builder 4 has been recently introduced to Adobe Creative Suite for the release of CS5. It is primarily a program designed to create interfaces that gather data (via forms) and present data (through views) from server databases.
Flash Builder 4 was formerly known as Flex Builder which was quite similar to Flash Professional. The closest CS5 relative of Flash Builder is Dreamweaver as they share a number of similarities in procedural definitions and binding server databases. Basically Flash Builder 4 is an upgrade to Flex Builder 3.
Flex framework is a popular open source platform that generates either MXML format or ActionScript 3.0 code. Though it is technically possible to code Flex applications with a free SDK, it could be quite difficult to execute a Flex coding project without Flash Builder. This is because Flash Builder offers useful tools like automatic testing, debugging hints and syntax hints for quality code development.
Tags: Flash Builder 4, Flex Builder