Update 3.x branch references to 4.x

[GrahamCampbell]

The recent merge from 3.x carried that branch's references to the 3.x branch name onto 4.x, where they should instead refer to 4.x. This points the continuous integration step that retrieves the base commit for git diff at 4.x, sets the package.json lint and prettier scripts that operate on changed files to diff against 4.x, updates the release process note to push to 4.x, and switches the documentation links embedded in the credentials help URL, the configuration validation messages, and the deprecation log output to the blob/4.x path, with the matching unit test expectations updated to agree. The referenced documentation pages all exist on 4.x, so the links resolve, and the test/README.md examples already use relative links and need no change.