Ubiquiti recommends using mongodb 3.6 with Unifi.

Unifi in pkgsrc can use either mongodb3 or mongodb4
- mongodb3 (the default, and the last 3.4 version of mongodb before
  the switch to server-side-public-license)
- mongodb4 (requires the 'new' server-side-public-license, and
  is not officially supported by Ubiquiti)

Recommended process of switching between them:
- Login to unifi, and download a full backup
- Shut down unifi
- Install new unifi version
- Rename away the old unifi/data directory (this also provides
  a rollback option if you need to revert) & create new empty data
  directory
- Start up unifi
- If you have customised data/system.properties for unifi.http.port
  and similar, shut down unifi (now that a new data/system.properties
  is present), merge the changes across in an editor then start unifi
- Go to the web interface and select upload backup