how to display ads after every 4th post

You’d need a counter in your loop. Something like this… (idea only, not tested!!):

$count = 0;

while (have_posts ()) {
    $count++;

    if ($count == 4) {
        // Ad code in here
$count = 0;
    }

     the_post ();
} // while ()

tried it buddy it does not work

what kind of ads?
always the same ad?

what theme are you using?

you might be able to do this by adding an action to ‘the_post’ –

https://codex.www.ads-software.com/Function_Reference/add_action
https://codex.www.ads-software.com/Plugin_API/Action_Reference/the_post

OK. This does work (I have tested it, and I know it works):

while (have_posts()) {
	the_post ();

	$count++;

	if ($count > 4) {
		$count = 0;
		echo "<p style='font-weight: bold'>Ad Banner Goes In Here</p>";
	} // if ()

	echo '<h1>'.the_title ().'</h1>';
	the_content ('Read More');
}

You do have to put it in the right template file for it to actually display, so take a look at where you’re trying to add it in.