It will install in it's own directory and won't overwrite anything with K3.
Your library will stay in place. Also, I'm not sure that the pianos aren't still in K4. NI has discontinued Akoustik Pianos as a product, but I think everything is still in K4 that was in K3(it was never the entire library, just parts of it).
But no worries. Just install and there will be no problems or conflicts.