Those who watch Only Connect will know how this works. For the others, here is a quick explanation:
You have seen three pictures, the first is here, the second is here, and the third is here. You have to try to predict the fourth picture in the sequence. Obviously this is (should be) easier the more pictures that you see.
And here is Picture 4
The pictures are a Padlock (or Lock), a Pit Prop (or Prop), a Prostitute (or Hooker) and finally, above, the Pit Prop (Prop) again. These are the names of the positions in Rugby Union worn by players in shirt numbers 4, 3, 2 and 1.