Interesting -- when you install the new upgrade/version, it would typically have given you the choice to work with the existing catalog and upgrade it. It would I believe have copied the catalog and worked with that (the old catalog would stay in your system until you decide you know longer want it).
So, the question is, when you installed LR5, did it pop up and ask you about the LR4 catalog? If not it seems strange!
At any rate, as Ken said, you can Import the old catalog and LR5 should pick up the LR4 processing. You could also try to just File/Open Catalog...there would likely be options as to handle things, look at each option that is given!
As to which is preferred, well, I'm not sure exactly how each approach is handled. One thing to consider: I believe if you go to LR4 and in the Library module select "All Photographs", then do a File/Export as Catalog... you then will get a separate catalog created which you then could either open in LR5 or Import as Catalog. That will keep the two catalogs separate.
I'm not sure how things would get handled if you just Open the LR4 catalog in LR5, so if you want to be sure of "keeping" the LR4 catalog untouched, then you could back it up before doing the LR5 Open! Lightroom has an option to back up the catalog "When Lightroom Next Exits" in the Catalog Settings dialog. Or, of course, you could just make your own copy of the LR4 catalog but rename it for LR5, keep it in the same directory, Open it in LR5, pay attention to the options, see what happens, report back here and be prepared to help others by sharing what you've learned!