I'm somewhat confused. From user PoV, the 4-th argument should be just a constant. Telling user that 4-th argument have to have size at all is already confusing. The fact that castColumn() should return a similar size const column is implementation details, — so what exactly are we checking when casting the 4-th argument with castColumn()?

Telling user that 4-th argument have to have size at all is already confusing

Why it's confusing? We can have the syntax like:

SELECT transform(arg_1, [arg_2], [arg_3], *)
FROM
(
    SELECT ...
);

Here we can see, what the size of the 4-th argument will be

The fact that castColumn() should return a similar size const column is implementation details, — so what exactly are we checking when casting the 4-th argument with castColumn()?

Your point is valid, I will check not the size of default_non_const, but the arguments[3]