There's an airside bus shuttle between gates A9, B23, and B54 which may be quicker than walking at T4 if you have to go between concourses A and B. There are also golf carts within concourse B. Concourse A is used for smaller jets, so you will both arrive and depart from concourse B gates and it likely will be quicker to walk or take golf carts if you are not particularly mobile and the distance between gates is far. Also, the widebodies don't use the gates at the end of concourse B past B41. 85 minutes should be more than enough time (MCT is 45 minutes). FYI, JFK-ATL is not a D1 route. You may be flying on D1 equipment, but the cabin will be marketed as First Class and have First Class service standards.

The bus shuttle stops are found on Level 1 of the terminal map.
https://www.jfkt4.nyc/terminal-map/