Today, Adobe released the official version of the Flex 4 SDK as well as the new Flex IDE/eclipse plugin named Flash Builder 4.
As with Flex Builder 3, there are two versions of Flash Builder 4: Standard & Professional. The main difference is Profiling and network monitoring features.
Purchase/upgrade costs are described here.
You can download the free Flex 4 SDK here.
You can download a free trial of Flash Builder 4 here.
*** BONUS ***
If you are a student or faculty member with a valid id as proof, you can get the Flash Builder 4 Standard for free here (for non-commencial use).