const regex = new RegExp('(.*?</p>){3}\\K', 's')
const str = `<p>If you wanna improve yer German, don't try to read Heine or some elevated crap... watch old episodes of [callout]Tatort or Bukow & Konig[/callout].</p>
<p>If I were teaching a music appreciation I wouldn't teach Beethoven. I'd teach Stamitz and average composers.</p>
<p>And here is a 3rd paragraph.</p>
<p>And here is a 4th paragraph.</p>
`;
const subst = `<blockquote class="pullquote">Tatort or Bukow & Konig</blockquote>`;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions