The current hotfix version is 4.2.1.0.
Unifi can be patched between releases by using a special Script Include called hotfix. If you find a bug in Unifi we may issue an update to hotfix so you can get the features you need without having to upgrade.
When upgrading Unifi, you can revert to the latest version of hotfix included in the upgrade. We reset hotfix with each release when the fixes become part of the main application.
We occasionally release a hotfix when minor issues are found. Simply replace the script in the hotfix
Script Include with the one shown below and you will instantly have access to the fixes.
These hotfixes will be shipped as real fixes with the next version of Unifi, so make sure you have the correct hotfix for your version.