Didn't you have a total of six witnesses?
Yep.
The first witness has a choice of 1 in 4 of choosing No.2.
The second witness has a choice of 1 in 4 of choosing No.2
And so on, for all 6 witness's.
The amount of options therefore is 4 raised to the 6th power.
4/4 to the 6th power = 1/4 to the 5th power = 1/1024
JohnM