Bump to release version 0.4.0

Bump version to full release and set release date in the changelog.

Author: st-bender

CHANGES.md

@@ -1,7 +1,7 @@
 Changelog
 =========
 
-v0.4.0 (unreleased)
+v0.4.0 (2025-08-15)
 -------------------
 
 ### New

src/spaceweather/__init__.py

@@ -8,7 +8,7 @@
 """Python interface for space weather indices
 
 """
-__version__ = "0.4.0.rc3"
+__version__ = "0.4.0"
 
 from .celestrak import *
 from .gfz import *