As an easy explanation:
T4: Shenghen flights.
T4S: non-Shenghen filghts.
But all IB flight to London operated with a narrowbody aircraft (e.g. 319, 320, 321) depart from T4. There are some gates in the T4 terminal that have non-Shenghen status.
If everything goes to plan, you will arrive in T4, disembark, check the screens and without changing terminals, go to the gate in the same terminal. It's very easy to navigate, The terminal is like a large corridor and on either side there are gates. There are also travelators so you move faster.
However, you may land in T4S - in that case you will have to follow directions for T4. You'll have to take a shuttle-train.