Perhaps you meant to ask about Flash Builder 4 and Flex Builder 3 co-existing? If so, then yes, you can also have Flash Builder 4 and Flex Builder 3 on the same machine without any conflicts; although I would not recommend installing them in the same eclipse instance.
You can also have as many Flex SDKs installed on your machine at a time and have Flex Builder 3 or Flash Builder 4 setup to use all the SDKs. Although, I don't thikn it is possible to use use more than one SDK as part of a single project. it'd be impractical, if not impossible.
I think that covers all bases.