It is ideal to take a backup of Catalog before proceeding with any sort of changes .
You may check with below link once regarding upgrading Catalog to from older version to Higher version.
After you install Lightroom from the creative cloud and start the program, the first thing it will do is find the catalog you have been using and prompt you to update the catalog. This will create a new copy of your catalog, and will include all of the images that you have imported now as well as all the work you have done on them. So that won't be necessary for you to import anything because it will already be done. Then you can start using Lightroom CC and all of its new features and benefits.
thank you both for that, I have LR4 installed on the mac book but everything to it is stored on a external hard drive. Can I simply let cc find my stuff the just carry on using it from the hard drive on my second compute?
If Lightroom CC can find your catalog file, it will ask you if you want to upgrade the catalog, you should select YES or OK or whatever the positive response is. If it can't find the catalog file to upgrae, you can force this to happen by File->Open Catalog and point to your Lightroom 4 catalog file.
After that finishes, you may or may not have to reconnect your photos.