Can't load Nttp in project with .net framework 4.0

[araad1992]

I have a project in .net framework 4.5 but for requirements this is neccesary in .net framework 4.0.

Now when I change to this version of the framework can't load the NHttp.dll appear this message

Unable to load the file or assembly 'NHttp, Version = 0.1.9.0, Culture = neutral, PublicKeyToken = 156364e4f7b202d9' or one of its dependencies. An attempt was made to load a program with an incorrect format.