The 4.0.1 MongoDB Java & JVM Drivers release is a patch to the 4.0.0 release and a recommended upgrade.
The documentation hub includes extensive documentation of the 4.0 driver, including
What’s New: what’s new in the 4.0 driver
Upgrading: upgrading from the 3.12.x driver
Installation MongoDB Driver: how to get the Java driver.
Installation MongoDB Reactive Streams Driver: how to get the Reactive Streams driver.
Installation MongoDB Scala Driver: how to get the Scala driver.
and much more.
You can find a full list of bug fixes here.