As discussed with metze on IRC, it might be good to give the overall design of winbind some more thoughts, as the differences in the samba4 architecture might call for a different design for winbind. It might be best to talk this over on SambaXP, where it's easy to include Volker and Günther, as well as tridge.
Jerry and Volker were discussing if the RPC bind was fully async. This needs some looking into.