When I was at the TFL/HEX station by the Central Bus Terminal a few Sundays ago, there was a human agent at the turnstiles handing out free passes to use for the Terminal 4 train. Be sure to keep the pass handy after boarding as you will need it to swipe out at Terminal 4. The train was running on a 30 min schedule that day so you might want to have an alternate plan on hand in case you are time-constrained.