I think you mean *Terminal* 4, not *Platform* 4.
AirTrain service is currently suspended from 9:00 PM until 7:00 AM, Sunday-Friday, so you should be able to still catch a train at 8:00 PM. Trains to Federal Circle normally use Track 1, but always check the destination signs -- or ask an attendant -- before boarding.
If, for any reason, AirTrain service is suspended by the time you are ready to take it, the Port Authority will be operating substitute shuttle-bus service from each terminal to Federal Circle.
I think you mean *Terminal*