Space under heading 4 is too wide

I am trying to change the space between the header 4 lines (booking, arrival, etc) and the body copy underneath. When I adjust the line height in the site style, it adjusts ALL of the header line heights making multi-line H1-H3 headers overlap (Like the words studio policies)

Googling + chatgpt are giving me this code, and I'm inserting it into the custom CSS space, but I'm getting an error on the first line. Any ideas how to fix, i don't know code really.

The site is not live and password protected, so I can't share the site right now. 

Thanks in advance

<h4 class="header">Header 4</h4>

<style>
.header {
  margin-bottom: 10px; /* adjust this value to decrease or increase the space */
}
</style>

Thank you Ziggy!