The two programs are completely independent of each other. Both versions can be installed on the same computer, and they will run independently. When you install Lightroom 6 it will prompt you to allow it to create an upgraded version of your catalog. This will create a new copy of your Lightroom 4 catalog, leaving the original intact and usable by Lightroom 4.
Thank you very much--I didn't like the idea of uninstalling LR 4. I've upgraded the CS versions of Photoshop without uninstalling, so it made no sense to nuke LR 4.