This the default behavior, I believe it will install to /Developer/XCode4 Just be careful not to install the extra system packages and only install XCode4. If you read the PDF that is linked on the same page as the preview it explains everything.
It'll install at '/Xcode4' and definitely make sure the "Unix Development" check-box is NOT checked before installing or it WILL overwrite your current installation!
If you're not a "paid" member: How did you get access to the XCode4 beta? Pirating pre-release software, then complaining that it doesn't work as expected... ts ts ts
As a paid member, I did not need to input any credentials; must be something to do with all the certificates that a paid Dev sets up for developing and deploying iPhone apps....
It should accept your normal developer.apple.com login credentials. If it doesn't, it's most likely a server problem on Apple's side (often happens around new XCode releases).
I use the organizer window in 3.2 for my ruby projects xcode 4 won't let me drag the projects onto the bar and edit them. Any idea what up with this?? thx