Announced a few days ago a new (major) version release of dev-lang/R, details at... https://stat.ethz.ch/pipermail/r-announce/2017/000612.html I've bumped locally on one machine (~amd64) and it builds and installs ok and runs everything I've tested so far no problems.
dev-lang/R-3.4.1 now released, bug submitted at https://bugs.gentoo.org/show_bug.cgi?id=623862
=dev-lang/R-3.4.1 is now in Portage.