Sigh...
Fine. Let us first examine the actual words used in Article IV, Section 3, Clause 1 of the Constitution:
"New states may be admitted by the Congress into this union; but no new states 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."
Now let us examine the actual words used in the Joint Resolution for Annexing Texas to the United States, which was expressly passed by the consent of both "the Senate and House of Representatives" (and not just by ratification of the Senate under the Treaty Clause):
"And be it further resolved, That the foregoing consent of Congress is given ... New States of convenient size not exceeding four in number, in addition to said State of Texas and having sufficient population, may [not "shall"], hereafter by the consent of said State, be formed out of the territory thereof, which shall [not "may"] be entitled to admission under the provisions of the Federal Constitution ..."
The drafters of the Resolution clearly understood the difference between the words "may" and "shall," having used them both in the same sentence. By passing this Joint Resolution of both Houses of Congress using the words "shall be entitled to admission," Congress expressly provided their consent, in advance, to Texas forming up to four additional states, in which case those four additional states "shall" be entitled to admission to the union.
In other words, if Texas at any time wants to split into a total of five or less States, then they have already gotten the consent of Congress. If Texas wants to split into a total of more than five States, then they have to go back to Congress and get additional consent.
Your interpretation of the Resolution not only changes the word "shall" to "may" but renders this entire section of the Resolution utterly meaningless. Under Article IV, Section 3 of the Constitution, any State "may" split into as many new States as they want, but only if they first get the consent of Congress, which Congress "may" or may not give. Congress has already given its consent for Texas to form up to four new States, in which case those States "shall" be entitled to admission to the union.
It is axiomatic in statutory and contractual interpretation that no term should be interpreted in a manner so as to render any provision superfluous or meaningless, or as Rush Limbaugh is fond of saying, "Words mean things."
Of course, this is all academic now, since the Constitution and statutes of the United States now mean whatever the hell five Supreme Court Justices want them to mean at any point in time. I can only hope that someday (perhaps in my children's lifetime) a majority of the Supreme Court Justices will be strict constructionists like Scalia, so that once again words will mean things.
Congress could not have fully consented to the division of the state because it wouldn't have fulfilled its obligation to ensure a republican (little 'r') form of government ("admission under the provisions of the Federal Constitution"). So in order to prevent Texas from dividing into additional states, all Congress has to do is withhold approval of the proposed constitutions of the new states.