As long as city mayors can manage city mayors, this scales up arbitrarily. A Level 4 cluster would have a prez running city mayors running city mayors running mayors running nodes. It handles issues that are unique for the cluster, namely the master copy of a database and naming services. Mayors manage nodes. Nodes are the computers that users access.
City mayors manage mayors or city mayors.
Thus, their functionality is to aggregate information from mayors and pass it up to their managing mayor, or to distribute information down the chain.