9 states claimed to no longer be part of the former Union, regardless of whether the other 4 ever ratified the Constitution.
The 4 states did not have to say they were outside the union. 9 of the thirteen states had left and the government under the AoC had dissolved. The union under the AoC ceased to exist. The 4 states had not yet joined the new union. 2 states had voted NOT to join the new union.
The 9 states could not lawfully change one word of the AoC without unanimous consent. They lawfully seceded and formed a new union.
Case in point: the laws in existance prior to the ratification of the Constitution remained in effect. Laws such as the Northwest Ordinances of 1784, 1785, and 1787. These are sometimes called "pre-Constitutional law," and are just as valid today as when passed. Some of the institutions of Government remained intact as well, such as the United States Postal Service, the Army, and the Navy. Ambassadors remained the same too.
It is easy to understand that Constitutional ratification was not largely viewed in the late 1780's as seceding from one country and forming another, but as a change in the way that the country conducted its business.
You are forgetting something:
“To begin with, the Constitutional Convention had been called by Congress, and Congress, without endorsing the Constitution, also approved its transmission to the states for consideration. Thus, as Madison argued in Federalist 40, the convention had some colorable claim to be operating under the authority of the existing government. Secession, on the other hand, was a wholly unilateral activity by individual states. Moreover, at the convention itself, Madison had pointed to the ratification process as a method of making the Union more solid than it had been under the articles. Because there were more treatylike, the Articles of Confederation were more easily broken. So far as the Articles “were to be considered as a Treaty only of a particular sort, among the governments of Independent states, the doctrine might be set up that a breach of any one article, by any of the parties, absolved the other parties from the whole obligation.” Hence, Madison said, it was indispensable that the new Constitution should be ratified in the most unexceptionable form, and by the supreme authority of the people themselves.” Thus, the Framers would have regarded secession under the Constitution as raising much different issues than under the Articles.”
The Tenth Amendment really adds nothing to Davis’s arguments. To the extent that the states had the power to secede prior to 1789, it was not because such a power was granted by the Articles of Confederation. On the contrary, the Articles purported to be perpetual and irrevocable. Rather, any such power existed only because, under international law, nations have the power to repudiate their treaties. That power is relevant only if the Constitution was a treaty between sovereign nations, as the compact theory insists. So the Tenth amendment argument is really only a restatement of the compact argument, rather than a source of additional support.
In basing his argument on compact theory, Davis built on a weak foundation. The evolution of American concepts of sovereignty was too complex and confused to support confident conclusions about the right to secede. Davis focused on one aspect of the record, ignoring a great deal of evidence with more nationalistic implications. For the argument to work, only did states have to retain some residual sovereignty after ratification, but they had to be the exclusive receptacles of sovereignty. Nether the text of the Constitution nor the weight of the historical record can support such a strong version of state sovereignty.”
--Lincoln’s Constitution, pp. 84-85 by Daniel Farber
Walt