3.4.2 is currently stable. 3.5.2 is currently unstable.
d95a1507936522f82f65e8ffc1937706cb0cd358 bumped mongodb to 3.4.2 already.
3.4.4 got pushed today ! C.J. I'm sorry but I cannot not invest time in maintaining an unstable version of mongodb on portage. I advise you to maintain a local overlay and have unstable version packaged as a copy of the stable ones instead. Thanks for your understanding.