I am wondering how to label a theorem in LaTeX using a letter but not changing the numbering of the other theorems. For example, I want Theorem 1.3 to be Theorem A and Theorem 1.4 to remain as Theorem 1.4 (not changing its numbering).
You can use an auxiliary counter to store the current value of the theorem counter, then perform the necessary redefinitions for the new numbering schema and then restore the saved value of the theorem counter:
It's not very clear why the theorem following "Theorem A" should be numbered 1.4. So I'll give you two distinct solutions.
The sequence will be 1.1, A, and 1.2
The sequence will be 1.1, A, and 1.3