Yes they are on standby until after the v4, this was a joint decision on the topic. The current maintainer of the Mongoose package will maintain our v3 connector after the v4 stable release during the 6 month security update time.
After the v4 release all of us (Strapi, Mongoose Maintainer, and the MongoDB team) will relay information to allow the latter 2 to build their own MongoDB connector and work with us on how to implement it.
This removes our team’s burden of trying to maintain the connector and have it built by a team that has a much better skill-set in noSQL databases. The reason for the delay is not just related to v4, though that is a large part since we haven’t even started working on the database layer for v4 yet (scheduled for early Q3), but also due to the other 2 team’s availability and planning.