A new Patchfinder utility with support for ArcGIS 10.4 builds has been posted to Esri support site:
Windows PatchFinder 10.1.2.1 (2016-02.09)
http://downloads2.esri.com/Support/downloads/other_/PatchFinder.exe
Linux PatchFinder 10.4 (2016-02-23)
http://downloads2.esri.com/Support/downloads/other_/PatchFinder.tar
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