You can't tell from the Serial #, put the CD in and you'll know. But you should not give away your Serial# like that.
I put the CD in my comptuer and the installation process started. As I have version 2.7 already installed,, I didn't want to take a chance of replaceing that one with an earlier version.
I really regret that Lightroom 4 cannot be installed on a Windows XP operating system.