"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."
Article IV, Section 3, Clause 1
What you say is true but, I believe Texas has the right to subdivide into 4 states, should it choose to, in accordance with its original charter accepting it into the union.
Texas can split into a total of five states actually. It was expressly agreed to as part of the Joint Resolution for Annexing Texas to the United States, Approved on March 1, 1845:
“New States of convenient size not exceeding four in number, in addition to said State of Texas and having sufficient population, may, hereafter by the consent of said State, be formed out of the territory thereof, which shall be entitled to admission under the provisions of the Federal Constitution ....”
Basically, the requirement under Article IV Section 3 of the Constitution, that Congress consent to the division of Texas, has already occurred.