Greasemonkey: change text in HTML

I have looked at several solutions for similar problems in here but I wasn’t able to understand either

The webpage I’m trying to edit has the following bit:

<h4 data-v-42b52bab="" class="title is-4 mt-4">Behemoth <!----></h4>

I’m trying to make it say “Plots 19, 20, 21, 49, 50, 51” instead of Behemoth

What I attempted was:

var strongElems = document.getElementsByClassName('is-4');

for (var i=0; i<strongElems.length; i++)
{
  var thisElem = strongElems[i];
   thisElem.textContent = "Plots 19, 20, 21, 49, 50, 51"; // change it
}

But it doesn’t seem to work. What can I do?