I've recently got a new laptop and installed creative cloud with Lightroom and Photoshop.
I've also got an old iMac with Lightroom 4 on it and quite a few photos.
Sorry if this is a silly question but if I install creative cloud on the old iMac, will I be able to 'merge' or update the old catalogue or pictures with the new version?
The first time you run the Creative Cloud version on the old computer, it will recognize the catalog you have been using and prompt you to update it. You should accept that and allow Lightroom to complete the task. This will create a new copy of the catalog in the format that the new version of Lightroom uses, and will have the same name except it will have a -2 appended to the end of its name. That means that you will now have both versions of the catalog on your computer. After the conversion is complete Lightroom will open and will display all the work you have done previously and everything should appear normal. Now you can delete the old version of the catalog and even remove the old version of Lightroom because the Creative Cloud version is not dependent upon it for anything.