There's no need to export anything. Open the LR4 catalog (.lrcat file) in LR 5, and it will convert and open a copy of the catalog.
And your photos aren't "in" Lightroom, they are referenced by the catalog, and all your edits are also stored in the catalog. The originals are not changed.
I did that and it opened in LR5 however I did not have access to the other LR5 photos. What would happen if I drag and drop the LR4 catalog into the LR5 catalog all from within LR5
What I ended up doing is opening LR5 and then open file and import from another catalog, I selected the LR4 catalog (.lrcat file) and ended up with everything in LR5. The original LR4 catalog still exists ( can this now be deleted? )