First of all, either only make an array of size 4 (or whatever size you are actually going to use) or split the array into arrays that have valid values (not null).
Then, in your second for loop where you split the array into the left and right side, your condition for the loop says that i < n - mid
but since you're starting at i = mid
you don't make the array large enough.
Change it to this and you will no longer get the exception:
for(i = mid; i < n; i++)
right[i-mid] = A[i];