We're using flex 4 beta 2 for our upcoming project.
I must admit that it's not yet production stable, there are bugs, and some of them are showstoppers. (we had some problems with data grids at least).
Though it's quite usable and works.
So, if your new project is a long-standing one and you're ready to update from time to time to the latest nightly, so go for flex 4. It's very likely that Adobe will release the final version in 2-3 months timeframe and you can easily switch to it.
If your only concern is a TLF-enabled text area, then consider as an option porting using TLF directly in your flex 3 project. That was the way I've chosen for my last project.
I've backported RichEditableText
component to flex 3 from flex 4. It wasn't that hard, cause it had little dependencies on flex 4.