Apple has pushed an update to the data used by Gatekeeper, bringing its version number to 164, dated 25 March 2019. This is in addition to the main 10.14.4 update, and should be installed after that, as a separate update.
Apple provides no details as to what changes this update brings, but it is normally expected to include recent revocations of security certificates used in signing software.
You can check whether this update has been installed by opening System Information via About This Mac, and selecting the Installations item under Software.
A full listing of security data file versions is given by LockRattler and SystHist for El Capitan, Sierra, High Sierra and Mojave, available from Downloads above. If your Mac has not yet installed this update, you can force an update using LockRattler, or at the command line.
I have updated the reference pages here which are accessed directly from LockRattler 4.2 and later using its Check blog button.