I was able to resolve this. An old .dll from a different plugin's directory made it's way in to the 4.0 project directory. It has been there since 3.8, but for some reason created no errors in 3.9. Regardless I have removed it and everything is alright now. Thank you for the help!
This error specifically is resolved. However, I am now getting errors that I am missing references to certain .dll files like System.Web.Mvc, Antlr3.Runtime, etc. None of these are used by Nop anymore in version 4.0, but for some reason they are used in the various plugins Nop Templates offers? If that is true, why aren't these dependencies bundled with the plugins? Am I thinking about this correctly or am I missing something?
None of our plugins in the 4.0 version use those dlls. Are you sure that this is not coming from other plugins?
Also, please check if you are using the 4.0 version of our plugins.
Hope this helps!