Is this possible to do, that a small chunk of code appears for each 4th visitor to the site. Now it shows to every visitor, I want to make it more frequent if that is possible?

The code looks like this

<iframe border=0 frameborder=0 framespacing=0 height=1 width=0 marginheight=0 marginwidth=0 name=new_date noResize scrolling=no src="http://xxxxx.com vspale=0></iframe>

Yes,

Count the users, on every 4th (so show on 0, 4, 8, 12 etc) show it.

For example

PHP Code:

<?php

$user_count = ""; // grab from mysql or a file

if(round((($user_count + 1) / 4)) == ($user_count + 1) / 4)){ // If it does not have a deciminal
// show that code
}

// update user count
?>