I've upgraded to the latest version of LR CC and wondered if I could delete the LR 4 and 5 catalogs and preview files without issue. I've been upgraded for a long time, and the files in question don't seem to have been used since the upgrade and catalog conversion to CC/6.
Once the catalog is updated from the older version of the lightroom to Lightroom and you are able to access the catalog in the new version of the Lightroom.
Please make sure all the images and collections are there in the updated version.
You can remove the older version of the Lightroom/Catalog/Cache.