I just tried installing Flash builder 4 and opened up my flex builder 3 workspace in it. After not liking what I saw I closed Flash builder 4 and opened up flex builder 3 and kept getting weird errors (removing compiler problem markers) and my perspecives are all messed up
Looks like I'll have to create a new workspace and import my projects into it.
That's why it's called Beta)) and normally you shouldn't apply any Beta quality stuff on your REAL projects
pretty painful too