You can uninstall Lightroom 4 if you want. It is your choice.
A slightly more verbose answer.
When you do a major version update, Adobe does a side-by-side install of the new version so that you still have the old version available if things go wrong. After you are happy with the way LR6 is running, you can uninstall LR4. (When LR6 opened your LR4 catalog, it migrated the old data to the new format LR6 catalog, leaving your LR4 catalog unchanged.)