Just as a bit of a PSA since I got to experience this twice/thrice yesterday and don't see a thread for it:

Due to "track issues" the EWR AirTrain is operating in two segments -- one segment serves all stops besides except the NJT/Amtrak station, the other serves only P4 and the train station -- must transfer at P4 to continue from one to the other. Oddly (IMO) both segments are using the same side of the platform which doesn't help with speed/efficiency since it seems like frequently an inbound train will have to stop somewhere on the track to wait for the other train to exit the station.

My perception is that the P4-AirTrain is running slower than normal and it seems that the ratio is somewhere around 2-3 "terminal" trains for each "train" train. Only a single train is running the shuttle route between P4 and the Train station so if you "just missed it" you would have to wait for it to complete the journey before you can try again.

First time arriving to P4 (about 07:30) there was no indication from anyone that the train wasn't continuing so basically everyone in my car wound up returning to C to try again; asked at C and was filled in -- second try at P4 someone mumbled 'last stop'; in the evening (~6:00 PM) it was a bit more chaotic but there were at least people making relatively clear and coherent announcements and somewhat forcefully trying to get people off at P4.

Per a couple of the redcoats on the platforms this "pinched loop" arrangement is expected to remain in place for ~6 weeks... so allow extra time, expect crowding, and perhaps help less frequent travelers with the navigation.