Autodiscovery should start from "lyncdiscoverinternal" and if it fails use external reference "lyncdiscover". According to my experience, S4B on-premise setup may have only "lyncdiscoverinternal" and no external discovery link.
Source: https://msdn.microsoft.com/en-us/skype/ucwa/rooturl - recommends internal discovery first
Autodiscovery should start from "lyncdiscoverinternal" and if it fails use external reference "lyncdiscover". According to my experience, S4B on-premise setup may have only "lyncdiscoverinternal" and no external discovery link. Source: https://msdn.microsoft.com/en-us/skype/ucwa/rooturl - recommends internal discovery first