Minimum mongodb version is 3.6 and that is ancient you will be fine with the latest of both Redis and MongoDB. You can use both if you want, select mongodb during installation and then add a redis block into your config.json to use it for session handling. We have examples at https://docs.nodebb.org/configuring/scaling/#configure-redis.
v4.0 didn't have a lot of breaking changes like v3.0 so all plugins that were updated for 3.x will work for 4.x.