New Framework K2

You can configure K2 to support .net framework 4.0 by doing the tollowing:

Modifying the "K2HostServer.config" file located in c:\program files\k2blackpearl\hostserver\bin folder.
Add the following key to the section of the K2HostServer.config file:

add key="netfxversion" value="v4.0.30319"
Restart K2HostServer.exe
a. You will be running in .NET 4 host, but will still execute .NET 3.5 code.
b. Although you can utilize .NET 4 based code constructs and references, you will not be able to use WF4 activities at this time

Ref: http://www.k2underground.com/forums/t/13211.aspx