A couple of major versions of Beautiful Soup 4 have been released since 4.6.3: https://bazaar.launchpad.net/%7Eleonardr/beautifulsoup/bs4/view/head:/CHANGELOG Just renaming beautifulsoup-4.6.3.ebuild gets 4.8.1 installed for me, although tests fail because 4.7.0 changed to use Soup Sieve to handle CSS selectors, https://pypi.org/project/soupsieve/ which is not available in Gentoo.
Added in: https://gitweb.gentoo.org/repo/gentoo.git/commit/dev-python?id=67298db5765054e4c318ef1322901b67043fc441