Unfortunately there is currently no way to have multiple versions of the Leap SDK installed on the same PC at the same time. The only way is to install the version that you are wishing to use at that time.
If anyone in the community has thought of a way to tackle this so you can play our desktop demos using V2 before switching to our VR demos which mostly use the V4 SDK then please feel free to share this here. This would be useful for me as I often need to test in all of our different software versions!