Yes SL4 = Silverlight 4.0
As for the namespaces, the primary namespace is System.ComponentModel.Composition there are some sub-namespaces as well and they all relate to MEF. Here is a quick overview of the namespaces of interest:
System.ComponentModel.Composition - Contains the attributes (Export/Import) and types needed to use the default attributed programming model and primarily used by component authors.
System.ComponentModel.Composition.Hosting - Contains things like the Container and Catalogs and primarily used by the application host author.
System.ComponentModel.Composition.Primitives - Contains the underlying building blocks that MEF is built upon and would generally one be needed by someone who wants to build a different programming model other than the attributed model.
There are some other misc sub-namespaces that are used for internal separation or advanced scenarios, generally unless you are doing something advanced the first two namespaces are the only ones you should care about.