Answer :

It will be seen in the illustration how the prisoners may be arranged so as to produce as many as sixteen even rows. 
There are 4 such vertical rows, 4 horizontal rows, 5 diagonal rows in one direction, and 3 diagonal rows in the other direction. 
The arrows here show the movements of the four prisoners, and it will be seen that the infirm man in the bottom corner has not been moved.

Math Genius