TYPO3 Core

Basically this is the extension authors fault. But I will add a way to override the check for version numbers before the final release.

" Basically this is the extension authors fault."

Not necessarily. The extension author has written his extension when not TYPO3 4.0 has been available. So he cannot have tested it with TYPO3 4.0 and therefore he cannot say that it would be compatible or supported with following versions.
So I think the extensions authors should always set the upper limit to the TYPO3 version under which they have tested it successfully.

Yes, this is also true. But whenever versions are part of dependencies, authors should update them as fast as possible... :)