Also remember that you can check for available patches thru the utility available with 10.4.x Server
Start->All Programs->ArcGIS->ArcGIS 10.4.x for Server->Check for updates...
Which will show what you have installed, and what else is available, for example
Those that are not yet installed will have a direct link to the download page for the patch. For those already installed, you would have to go to Esri Support Downloads to get additional information list like Avinash posted.
Just some additional info so others don't miss other available patches.