I believe the answer would be 4!
So 4 * 3 * 2 * 1
24.
Here's proof :
ABCD
ABDC
ACBD
ACDB
ADBC
ADCB
BACD
BADC
BCAD
BCDA
BDAC
BDCA
Following this order, each letter has 6 additional options, so 4! Would answer this appropriately. Some would say 6*4 but that's the same as 4*3*2 (3 and 2 becomes 6)
