Assume you are in T4.

Just walk back towards the central area, where security is. When you see Gucci on the left, there is a walkway to the right, opposite Gucci, marked "To Multi Faith Prayer Room". Apparently that is code for "Exit".

Walk this way, then downstairs, ignoring any no entry signs (they are code for "This Way"). Don't go all the way down to the buses (connecting passengers from other terminals), but in to the security area to the right (where passengers arriving at T4 go, to transfer to other terminals). To the right is a turnstyle gate, that brings you into the Arrivals area.

From there, go through passport control, down to the (no) baggage hall and out.

Alternatively set off a fire alarm, and you can simply leave the terminal with everyone being evacuated.