I completely disagree, see my comment above. I was doing asynchronous distributed data replication and compute over a grid in 2001 using Jini and Javaspaces, and I was the one late to the party.
Moreover, Sun’s legacy in distributed system goes far beyond theory, people forget about Jini (and Javaspaces) a distributed computing platform that shipped by 2000 or before (I had the pleasure to work with and it was the only reason I would develop in Java), SOA? micro services? In-memory distributed computing? It was all there at work. Want P2P?, enter JXTA. There you go.
The problem with this approach is that the 4 Crossbar instances will need to have a connection/session with at least an instance of each service you want to have global visibility of. This is because WAMP is a session-based protocol, every peer needs to be connected to a Router in order to interact with other peers. A Router cannot initiate sessions, only clients can.
Tomorrow I am presenting Bondy (our OSS WAMP router implementation) in the Riak London Meetup (https://www.meetup.com/riak-london/events/250813756/) and it will be available in Youtube later. As opposed to everything you might have seen, BONDY works in cluster. It is written in Erlang and uses eventual consistency model where data is replicated throughout the cluster using a gossip algorithm (plumtree - epidemic broadcast trees http://asc.di.fct.unl.pt/%7Ejleitao/pdf/srds07-leitao.pdf). Bondy embedds an HTTP/REST API Gateway and will provide an MQTT gateway in the future.