Sat 4 should be coming, except.....no one is going to pay for it. Remember STN was built as London's new airport with all the bells and whistles of government backing. Ryanair will not cough up the cash, they don't even want to pay for the train to Sat 1 and 2 which is why there's a walkway to Sat 3. With easyJet and Jet2 being the other major customers, I suspect Sat 4 if it ever arrives will look nothing like the first three. It will be like a bigger version of the end of Sat 3.