insert the following line into /etc/make.conf
PYTHON_ABIS="2.7 3.4" PYTHON_SINGLE_TARGET="python3_4"
and unmask python 3.4, either with
emerge -auDN @world --autounmask-write
etc-update
or just edit the /etc/portage/package.unmask and insert the line your self
then update your world
emerge -auDN @world
[Update:] added PYTHON_SINGLE_TARGET
to make.conf