transfer of a post automatically

Hello I want special programming for Wordpress, a transfer of a post automatically after a certain number to another category has to be chosen when the post is written

I want when the post is the arrival of No. 4 (test1 post) show entry in the category of No. 2 category is determined No. 2 when the post is written photo http://i.stack.imgur.com/dzeXh.png