Say that the letters are A,B,C, & D. Write out all 24 permutations of ABCD. You will see that there are only two cases in which A alone is in its correct place. You will also find that there only cases two having B alone in its correct place. The same is true for C&D.

ABCD

ABDC

ACBD

ACDB

ADBC

ADCB

BACD

BADC

BCAD

BCDA

BDAC

BDCA

CABD

CADB

CBAD

CBDA

CDAB

CDBA

DABC

DACB

DBAC

DBCA

DCAB

DCBA