Replace first <p> tag using HTML DOM

Q : As per task i need to replace <p> tag only for first occurrence with h4 using HTML DOM.

I try it using this question but can't success

My code,

HTML

<div class="productabcont">
    <div class="protabtop">
        <div class="protabtopright">
            <p>Combed compact yarn</p> <!-- This i want to replace with <h4> tag. -->
            <p>Description line 1</p>
            <p>Description line 2</p>
        </div>
    </div>          
</div>

PHP

<?php    
    $dom = new DOMDocument();
    $dom->loadHTML($myHTML);
    $dom->getElementsByTagName('p')[0]->outertext = '<h4>'.$value.'</h4>';
    $dom->saveHTML();
?>

Required Output

<div class="productabcont">
    <div class="protabtop">
        <div class="protabtopright">
            <h4>Combed compact yarn</h4> <!-- This is replaced with <p> tag. -->
            <p>Description line 1</p>
            <p>Description line 2</p>
        </div>
    </div>          
</div>

Thanks in advance.