Upddating to office libre 4, do I have to unistall 3?
When you run the LibreOffice 4 installer, it will uninstall any 3.x versions on your system before proceeding with installing 4.0.
But profile will be migrated, so if you made some settings they will be preserved.
You do not have to uninstall previously-installed version of LibreOffice if it is version 3.4.5 or higher. Lower versions should be uninstalled prior to installation. If you do have an existing installation of LibreOffice, all your preferences will be preserved and that old installation will simply be overwritten.
Source: Deployment and Migration