.NET 4 continues with the new concept of profiles. A profile is a subset of the Full Framework that can control what run time assemblies you can use. This can allow for a smaller install when redistributing .NET. For more information check Jossef Goldberg's blog - http://blogs.msdn.com/jgoldb/archive/2009/05/27/net-framework-4-client-profile-introduction.aspx
Also see my new blog on deployment which discusses the new .NET 4 Client Profile.