This may be documented somewhere but I did not find it. Also, those not familiar with Eclipse may not understand the terminology. So, here is how to install ColdFusion Builder and Flash Builder 4 together in 7 easy steps. These are written based off my experience when installing on both Windows 7 and MAC OSX. There are probably other ways to do this but this worked for me.

1: Download ColdFusion Builder. (for your OS)
2: Download Flash Builder 4, Plug-in version. (for your OS)
3: Run ColdFusion Builder installer and install as stand alone installation.
4: Close ColdFusion Builder if it started up post install.
5: Now run the Flash Builder installer. When it asks if installing as a plug-in or stand alone choose plug-in. In the directory selector browse and choose the ColdFusion Builder install directory. Complete the installation.
6: Open ColdFusion Builder.
7: After Builder is open choose window -> Open Perspective -> other. On the list should be "Flash", select it and click open.

You now have Flash Builder and ColdFusion builder running in the same IDE. If you quit Builder and reopen it will reopen your previous perspectives. If you are running the trial versions you will get a license warning from both perspectives.

Hope this helps.