brew install mongodb@3.4
MacBook-Pro:~ catherine$ brew search mongo
==> Formulae
mongo-c-driver mongo-orchestration mongodb@3.0 mongodb@3.4 mongoose
mongo-cxx-driver mongodb ✔ mongodb@3.2 mongodb@3.6 percona-server-mongodb
==> Casks
jeromelebel-mongohub mongodb-compass-community mongodbpreferencepane nosqlbooster-for-mongodb
mongodb mongodb-compass-isolated-edition mongoproc orelord-mongodb
mongodb-compass mongodb-compass-readonly mongotron
catherineui-MacBook-Pro:~ catherine$ brew search mongodb@3.4
==> Formulae
mongodb@3.4
MacBook-Pro:~ catherine$ brew install mongodb@3.4