An article 4 free inhabitant is a reference to the articles of confederation, which is an agreement between the states that predates the constitution. The articles of confederation are fairly brief and it starts with a preamble which states that the states are agreeing to a set of rules to govern the union of those states.
This means that only people from the 13 original colonies would get to call themselves article 4 free inhabitants. However, most people feel that the current constitution overrides the articles of confederation and should therefore not apply.
Articles on Free Inhabitants in the United States
Article I. The Stile of this Confederacy shall be “The United States of America.”
Article II. Each state retains its sovereignty, freedom, and independence, and every Power, Jurisdiction, and right, which is not by this confederation expressly delegated to the United States, in Congress assembled.
Article III. The said States hereby severally enter into a firm league of friendship with each other, for their common defense, the security of their liberties, and their mutual and general welfare, binding themselves to assist each other, against all force offered to, or attacks made upon them, or any of them, on account of religion, sovereignty, trade, or any other pretense whatever.
Article IV. The better to secure and perpetuate mutual friendship and intercourse among the people of the different States in this union, the free inhabitants of each of these States, paupers, vagabonds, and fugitives from justice excepted, shall be entitled to all privileges and immunities of free citizens in the several States; and the people of each State shall have free ingress and egress to and from any other State, and shall enjoy therein all the privileges of trade and commerce, subject to the same duties, impositions, and restrictions as the inhabitants thereof respectively, provided that such restrictions shall not extend so far as to prevent the removal of property imported into any State, to any other State, of which the owner is an inhabitant; provided also that no imposition, duties or restriction shall be laid by any State, on the property of the United States, or either of them.
Since the article says fee inhabitants shall be entitled to all privileges and immunities of free citizens in the several states, it means you cannot put restrictions on citizens of other states. Also, a state cannot write rules to favor its citizens to the exclusion of citizens in other states of the USA.
The article 4 of the articles of confederation sees an agreement on some rules for dealing with people from other states but no definitive class of people as being free inhabitants with all the rights and protections of a citizen but no obligations. Nothing is establishing the right to travel and you cannot be restrictive of people from other states. So if one requires a driver’s license in that particular state, then everyone driving in that state will need one.
In conclusion, the article 4 free inhabitant does not hold any particular meaning and just happens to be a term used in Article 4.