When trying to add a new MVC4 project to the solution, I received the following error message:
Cannot add the item because the item to add it to is not a solution folder or the solution
To fix this issue, go to tools/options, click Show all settings, and in projects and solutions, just check the “Always show solution” option. Now in the solution explorer, your project will appear under its parent solution. Then you will successfully add a new MVC4 project to the same solution.