We have a large .NET web application that was MVC3, using T4MVC. We recently upgraded it to MVC4, and all is good except for T4MVCExtensions (version 3.6.5 from NuGet) is s
To isolate, I would try creating a tiny dummy assembly that is built against MVC3 to see if it behaves similarly. There shouldn't be anything special about the T4MVC helper assembly.
Also, please look at my test project https://github.com/davidebbo/T4MVCSampleApp. This is an MVC4 project that uses the latest T4MVC, and it runs fine for me. Does it run for you?
If all else fails, if you can share a repro project (e.g. on github), I can try to take a look.