That's about it. Oh, and I don't want to convert the Flex Builder Project making it incompatible with Flex Builder 3 Pro.
Yes. Adobe products are extremely backwards compatible. However, some Adobe products will by default save your "older" files into the newer format - ie if you saved your FB 3 project with the FB 4 application, you may have to choose "Save As" to force it to save it as version 3 (this is true at least for Flash, not certain if Flex is different in that regard). This is only relevant if you need to retain the older file format for compatibility with other coworkers who only have an older version of the program.
Yes! Here is an article and video that shows how to use the Flex 3 SDK in Flash Builder 4.
Yes, you can compile the project with Flash Builder 4. ISTR it either silently updated my project settings or I might have converted the project.
In any case, if you are trying to simultaneously use Flex Builder 3 and Flash Builder 4 you should note that at least one of the internal eclipse & FB project settings files (.actionScriptProperties, .flexProperties & .project) changed format between FB3 and FB4 - I can't remember exactly which ones changed off the top of my head.
This will affect your development workflow.
I have previously used Flash Builder 4 on a project whilst the rest of the team was using Flex Builder 3. Whenever they updated their project I had a lot of problems with conflicts in at least one of the above files. Whenever this happened, I just accepted the FB3 versions of the files, and let FB4 internally deal with upgrading the format. If I somehow needed to make changes to those files (such as adding a new class), I'd either get another team member to do it, use FB3 to add the files, or just manually edit the file myself. Irritating, but not the end of the world.
YMMV.