The devmanual should document EAPI 4 USE dependency defaults eg. app-foo/bar[baz(+)].
It does. Metadata use dependencies default In addition to the use-deps specified in EAPI=2, a (+) or (-) may be added to the use-dep to define a default-value in case the use-flag does not exist in the given package. The (+) means that this use-flag is assumed to be enabled, (-) the opposite. Example: <snip> http://devmanual.gentoo.org/ebuild-writing/eapi/index.html