Which sentences are punctuated correctly?

Question: Which sentences are punctuated correctly?

(1) when separating items in a list, a writer should end the list with a colon.

(2) separate the items with colons.

(3) add a comma after the “and” before the last item.

(4) add a comma before the “and” before the last item?

Answer 4 is absolutely right answer because we know that add a comma after the “and” before the last item is correct.