A fourth governmental function is to protect us from other governments. One part of this function, defense against external attacks, may not always be necessary. If there were a world government, external defense would be unnecessary. But the fourth function would remain, for it includes defense against potential governments that might try to replace a current one.
It is quite possible for two or more governments to rule over the same area at the same time. In the United States at least three governments usually legislate for any one place: the federal government, a state government, and a city or county government. Often there are many more than three. During revolutions several governments generally exercise some control over the same area. In 1917 Russia was governed from February to October both by the Provisional Government under Alexander Kerensky and by the Petrograd Soviet, dominated by Lenin. Both centers of authority issued orders that were obeyed to some extent. Likewise, large areas of South Vietnam were under both the Saigon regime and that of the Vietcong during much of the time the United States was involved in the war there. In many villages it was said that Saigon ruled by day, the Vietcong by night.
Subjection to more than one government is tolerable, however, only if there is a clear pecking order among them, as in the United States. When one government makes an action illegal and another in the same places makes avoiding that action illegal, their subjects are damned if they do and if they do not. They cannot protect themselves against sanctions for obeying the law, for obeying one law requires violating the other. The Supremacy Clause of the US Constitution is designed to avoid placing citizens in exactly this predicament:
This Constitution and the laws of the United States which shall be made in pursuance thereof and all treaties made ... under the authority of the United States, shall be the supreme law of the land, and the judges in every state shall be bound thereby, anything in the Constitution or laws of any state to the contrary notwithstanding.http://www.opentextbooks.org.hk
A government unable to protect its subjects from the sanctions of other or would-be governments is, to say the least, not a completely viable one.