Well, last year I flew Delta ORD-JFK, went through security at ORD, and landed at JFK Terminal 2. My Alitalia (SkyTeam partner) flight to FCO (Rome) was to depart from Terminal 4. I very much DID have to go through security all over again at Terminal 4, dragging my bags outdoors along the curbside. I had a 3-hr layover, and when I called Alitalia the day before, the customer service rep swore up and down that would be plenty of time, otherwise they'd never have allowed me to book such an itinerary online. My ticket was Premium Economy, and I had bid to upgrade to Business. Alas, my bid was rejected as too low.
Well, due to sleet, my flight from ORD-JFK was delayed more than 2 hrs, and I had only about 40 min. to walk to Terminal 4 and go through security. Ahead of me were a dozen or more families on a tour, pushing multiple strollers with squalling babies & toddlers. It took so long that while I was in the TSA line, I heard my name paged to claim my upgrade to Business--my bid was accepted after all as a standby; but by the time I cleared security and ran (OK, limped as fast as I could) through the terminal to my gate, not only had my upgrade been given away, but I was so late my original seat had been given to a standby passenger. I was placed on standby for Premium Economy. My boarding pass was reissued with only a couple of minutes to spare before the gate was to close--and the passengers & crew were annoyed with ME!
It's NOT true that Delta passengers can take the tarmac jitney from airside Terminal 2 to airside Terminal 4. Believe me, I tried. They CAN take it from Customs & Immigration at Terminal 4 and connecting to Terminal 2 without re-clearing TSA; but only crew and airport personnel can take the jitney from 2 to 4 without going through TSA again. (Of course, it doesn't matter if my return layover is too short to make my flight to ORD, as there are several more every day).
And in a couple of weeks, I'll be flying Delta on Prem. Economy from ORD-JFK, and then from JFK-AMS. I used a travel agent this time, and he said that both eastbound legs used Terminal 4 at JFK. Therefore, I have only a 2-1/4 hr layover, but that should be enough time since I'd only be switching gates and not going through security. Or so I thought. This afternoon, Delta switched the JFK arrival to Terminal 2 (as of this morning it was Terminal 4), so I have an even tighter connection to Amsterdam than I had to Rome last year.