“SharePoint 2010 does not support access from .NET Framework 4 or being run in a .NET Framework 4 app domain. If you must support the .NET Framework 4, we recommend that you develop a web service that uses the .NET Framework 4 and then connect to it from SharePoint. All server-side customizations directly for SharePoint 2010 still need to be either the .NET Framework 3.5 or the .NET Framework 3.5 SP1.”
How to enable assembly bind failure logging (FUSION) in .NET
"Add the following values to HKEY_LOCAL_MACHINESOFTWAREMicrosoftFusion Add: DWORD ForceLog set value to 1 DWORD LogFailures set value to 1 DWORD LogResourceBinds set value to 1 String LogPath set value to folder for logs ie) C:FusionLog Make sure you include the backslash after the folder name."
http://stackoverflow.com/questions/255669/how-to-enable-assembly-bind-failure-logging-fusion-in-net