I have juste purchased the full stand alone version of Lightroom 6.
Do I have to uninstall Lightroom 4?
Will I loose anything such as libraries or collections by doing so?
NO, you don't have to uninstall previous versions of Lightroom.
Nothing should get lost. The first time you launch Lightroom 6, it should ask if you want to upgrade your Lightroom 4 catalog, and you should answer YES or OK or whatever the affirmative is.
Thanks! I will that.