When Sitefinity 4.2 is released, you will be to install it in separate folder for example if you have upgraded to 4.1 from 4.0, you can see that the folder Sitefinity 4.0 still sits in your Program Files/Telerik folder as well as 4.1.
In our future release you can expect the same - another folder, called Sitefinity 4.2 alongside the old releases of Sitefinity.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items