(1,1)
(2,2)
(3,3)
(4,4)
(1,2)
(1,3)
(1,4)
(2,1)
(2,3)
(2,4)
(3,1)
(3,2)
(3,4)
(4,1)
(4,2)
(4,3)
所以有4×4=16对