I am looking to merge multiple columns into one list. I have some code that will allow me to merge three columns into one list, but I am looking to be able to merge more than three. below is an example of what I am trying to do, and below that is the code that I am currently using for merging three columns.

list1 list2 List3 merged list
AA AB AC AA
AB
AC

=IFERROR(INDEX(List1, ROWS(D1:$D$1)), IFERROR(INDEX(List2, ROWS(D1:$D$1)-ROWS(List1)), IFERROR(INDEX(List3, ROWS(D1:$D$1)-ROWS(List1)-ROWS(List2)), ""))) + CTRL + SHIFT + ENTER