They don't need to be told that information. They can deduce enough information on their own.

Prisoner 1 knows he is prisoner 1 because he is the only one who can see exactly two prisoners in front of him. Prisoner 2 knows he is prisoner 2 because he is the only one who can see exactly one prisoner in front of him. That information allows one or the other of them to solve the problem.

Prisoners 3 and 4 don't know which one they are, because both can only see a wall in front of them. But that doesn't matter, because they can't possibly know the answer anyway. They are red herrings in this problem.