The hunk of the AirTrain between Terminals 1 and 4 is closed, and will remain so for months. They'll be running a shuttle bus instead. This would affect those who, for instance, are connecting Air France and Delta flights, who might want to budget more time for the connection. Official announcement and a brief FlyerTalk discussion linked below.
https://www.jfkairport.com/announcements/airtrain-station12-closures
https://www.flyertalk.com/forum/new-york-city/2119908-jfk-terminal-1-airtrain-closure.html