Implement pygame.math.Vector4 - #3960

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Inline comments:
In `@buildconfig/stubs/pygame/__init__.pyi`:
- Line 49: Update the stub generator source or discovery input that produces the
pygame __init__ exports so Vector4 is emitted alongside Vector2 and Vector3; do
not manually preserve the generated line. Regenerate the stubs with the standard
generator and commit the resulting generated output.

In `@docs/reST/ref/math.rst`:
- Around line 1221-1222: Update the Vector4.normalize() and
Vector4.normalize_ip() documentation to state that normalizing a zero vector
raises ValueError, matching the documented behavior for Vector2 and Vector3.
- Around line 1149-1152: Update the Vector4 unsupported-method statement in the
Vector4 documentation to remove slerp from the listed unavailable operations,
while preserving the references to cross, rotate, and coordinate-system
conversions.

In `@src_c/math.c`:
- Around line 3873-3911: Run the repository’s formatting command, python3 dev.py
format, and commit the resulting clang-format changes for the modified method
table block containing vector4_reduce and vector___round__. Do not make
unrelated source changes.
- Around line 3751-3756: Update the Vector4 constructor and update()
keyword-argument handling so the zero-vector branch is used only when all four
components are omitted; route partial inputs such as y, z, or w alone through
the existing partial-component error path instead of silently discarding them.

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