The Burrito Preview installs to a different directory than your existing FB4 installation, by default, so there should be no conflict there. Also, it accepts the same Serial Number you have for FB4 if you purchased FB4 as an individual product.
If you purchased FB4 as part of a CS5 Suite, there will be a site available you can use to obtain a Preview SN for Burrito, but you can still use Burrito in Trial mode until that is available.
What does preview release mean? Should I keep my existing install of Flash Builder or is it okay to uninstall?
The Preview release is equivalent to what Adobe used to call a Public Beta.
As a Beta release, we recommend that you use it for checking out the new features under development, but not for Production use. We recommend keeping your release version of FB4 installed for Production use.
You can have FB4 and Burrito installed side-by-side, as they're designed to co-exist peacefully.