Set identity to several ZMQ servers, binded to same port

I am implementing a ZMQ DEALER-ROUTER pattern in Python 3.4. This code from ZMQ guide is the core of my program.
It sets identity for each client easily (ex. 1,2,3,4). However, if I start the same number of server workers, they are not paired with clients. One server worker may respond to clients with different identities.
My question is: how do I set identities for each server worker, so that server worker 1 interacts only with client 1? More precisely, I need 4 clients interacting with 4 server workers via one single port in a paired manner.