I have posted to the CodePlex site at http://exceldna.codeplex.com/ an update to Excel-Dna that adds support for .Net 4.
To load the .Net 4 runtime, just add a RuntimeVersion="v4.0" attribute to the DnaLibrary tag in your main .dna file. There are two small samples that show how this works in the Distribution\Samples directory of the .zip file.
By default, if no directive is present, the .Net 2.0 runtime will be loaded, so existing add-ins should work perfectly with no changes when using this version of Excel-Dna. An Excel session can simultaneously host add-ins that use .Net 2.0 and add-ins using .Net 4 without conflict. You can check which version of the runtime your add-in is running under by checking the value of System.Environment.Version.
I have also added an option to enable shadow copies on the AppDomain that is created for the add-in. To set this, add a ShadowCopyFiles="true" attribute to the DnaLibrary tag. I have not tested this feature yet.
As usual, please post any feedback, questions or problems with these
features to the Google group at http://groups.google.com/group/exceldna.