.NET 4.0 DLL cannot be added to VS Toolbox
It is a known issue that for some unknown reason, in Visual Studio toolbox, .NET 4.0 assembly is not getting added. But, it should work for other .NET framework like .NET 3.5 or 4.5. So, if you have a tight requirement that. only .NET 4.0 can be used, you may try adding the .NET 4.5 dll to your toolbox and when you dropped the control into your form, you can remove the reference from your project and read the reference from the path where the .net 4.0 dll is located.