Inserting content after WP in-content paragraph (not really tested)

gistfile1.php

$content = get_post( 1558 );
$content = $content->post_content;

$content_array = explode( "\n", $content );

echo 'before<pre>';
print_r($content_array);
echo '</pre>';

foreach ( $content_array as $k => $v ) {
	echo $k . ' ' . strlen( $v ) . ' </br></br>';
	if ( 1 == strlen( $v ) || '<!--more-->' == $v ) {
		unset( $content_array[ $k ] );
	}
}

$content_array = array_values( $content_array );

array_splice( $content_array, 3, 0, '<div>sdf</div>' );

echo 'after<pre>';
print_r($content_array);
echo '</pre>';