Fourth parties are to customers what third parties are to vendors. They are companies that work primarily for customers, and make most of their money by helping customers buy, rather than helping vendors sell. As customers acquire more native ability through VRM tools, it is only natural that fourth parties will arrive to help customers use those tools, and to add services that amplify the power of those tools and their users.
The idea of fourth parties came up during conversations within the VRM community. It is unpacked several places, but at this writing the best example is this post on the ProjectVRM blog.