I know I can add an adsense code between *every* post by adding the code to the header or footer of every post in POST/PAGE INFO ITEMS, but that is too spammy and I just want one ad between posts. Maybe between the first and second, or just before the 4th post only. How can I add a space for an adsense code only before one post, but not all? Thanks.

Yu will have to edit index.php and put logic in the loop to do that