I have tried creating an ebuild based on mongodb-4.0.12 myself, but because version 4.2.0 now requires Python 3.7 to build (see https://github.com/mongodb/mongo/blob/master/docs/building.md), it is probably not a trivial update.
I have submitted a related pull request for app-admin/mongo-tools. These tools are written in Go and building version 4.2.0 of them works as it did with earlier versions. If you're wondering "What good would the tools do without the MongoDB core?": I built the latter locally from the source code, but I was still missing the tools. Now there is less pressure for me until dev-db/mongodb is officially updated.
Just wanted to give a heads-up here. When 4.2 got released, I tried but failed to quickly update the ebuild with the updated build system of 4.2... This will require more time and for now I do not have it I'm afraid
I got it working :) enjoy
