The license handling code has not changed in 4.0.
Maybe the fact that we supply two dlls: one built for .NET 1.1 and the other for .NET 2.0 is relevant because we ship them in bin\net1.1 and bin\net2.0 folders.
For more info see http://www.aspose.com/Community/blogs/roman.korchagin/archive/2006/09/05/55897.aspx
If you placed you license in C:\Program Files\Aspose\Aspose.Words\bin then there is no Aspose.Words.dll in that folder anymore and therefore it will not find the license.
By the way, if this is the case, it is probably not a good place for your license anyway. It is not good because this folder is only on a developer's machine when you installed Aspose.Words.msi, it will not exist on the client's machine when the project is deployed.
You should keep your license together with your project. I mean inside the same bolder bin\Debug or bin\Release of the application you are building. Better yet, add the license to your project as an embedded resource, this way you will not have to worry about the license file location at all.