The first question would be, have they not already been consolidated?
When you installed LRcc it may or may not have created a new empty LR6 catalog.
When you went to open your old LR5 catalog, LRcc would/will prompt you to migrate your LR5 catalog to the new LR6 format.
LRcc would/will do the same thing if you attempt to open your LR4 catalog.
LR5 would have prompted you to migrate your LR4 catalog to the then new LR5 format.
How much, if any, of this data migration have you done over the years?
Thanks ManiacJoe! Never got the prompts to migrate catalogs. I have never done this over the years -- I'm only a clumsy amateur! Cheers!
The first thing I would do is to migrate the LR4 and LR5 to the new LR6 formats by opening each and migrating the data.
After that, from LR6, open the catalog that will be your master LR6 catalog, then use the command /file/import-another-catalog to add the contents of the newly converted catalogs to your master catalog.