NET 3.5.1 is still a required component for K2. Since .NET 4.0 is different to 3.5 you need to install it as well as .NET 4.0 if you are going to use VS2010.
There is a config file update you need to do manually after installing the VS2010 components for K2. This will make K2 to load the 4.0 runtime. Simply add an appsetting in K2HostServer.config:
K2 is still the only vendor to support .NET 4.0 inside the workflow engine along with other framework versions.