Save your image file to an external drive. Copy them over to an external drive.
Also copy over the LR 4 catalog file. It ends with .LRCAT
Check your Downloads folder for any and all install files (Hopefully you didn't delete them once the program was installed. A Very Bad Idea but normal for Mac people) (Like they'll never need them again. Hey I have a Mac).
Copy all of them over to that same external along with the folder they are in, IE Downloads.
Please note that LR 4 doesn't work well or at all on the newest version of OS X. Old program on a New operating system.
If the drive you now have in your Mac is still functioning make a Clone of it using Carbon Copy Cloner or Super Duper to a Second, Other, external drive than the one you used to backup all your files.