Section. 3.
New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States; and nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State.
Section. 4.
The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.
I’m not your lawyer. This is not legal advice.
Section 3 gives the process to add new states. First, it is a Congressional act. Second, it effectively requires the consent of the State. Congress can’t just stick a “Middle Carolina” in there, decide we need a East Virginia, actually, or that the Dakotas could just as easily be a single Super-Dakota, unless those States agree to that. It’s also phrased in a way that suggests that the States also can’t split up or recombine unless Congress gives its blessing. Wyoming gets two Senators. They can’t just split up into seven little Wyomingian satellites and suddenly get fourteen.
Secton 3 also gives Congress power over the territory and other property of the United States that is not the States. Then, it goes ahead and tacks on a little rider about the Constitution not prejudicing claims of the federal or state government.
Section 4, then, is hot out of the gate, saying that the Federal government has to guarantee a republic-style government to the States, must protect them against invasion and domestic violence (pending legislative action, or executive action if the legislature is busy).
Next week, we get to Article V, which is, believe it or not, actually pretty cool.