Joomla 4 is using FA5 icons see:
https://fontawesome.com/v5/search

Did you actually get this working chaps? I have the same problem in Nina on J4.
(Using wrench as an example) I've tried fa fa-wrench; fas fa-wrench; fa-wrench

None of them are doing it for me. Id be grateful for your assistance

L

Hello,
I just tried on J4 Nina Template to add fontawesome icon to a heading.

I took this code:

<h1><span class="fas fa-wrench"></span>Cookies</h1>

The result is
.


(to add some space around the icons I always add something like this to my custom.css:

.fa, .fab, .fad, .fal, .far, .fas {padding: 0 5px 0 2px;}

)

So in the J4 templates you should use the fontawesome icons with SPAN or I tag. There should nothing be in between the opening and the closing tag.

In J3 templates you could add a FA icon to a link like this:

<a href="#" class="fa-envelope">Link</a>

In J4 templates you should use it like this:

<a href="#"><span class="fa fa-envelope"></span>Link</a>

If you need a special FA icon, check your font-awesome.css in templates/j51_xxx/css/font-awesome.css if it is listed there.
Searching for icons in general is easy on https://fontawesome.com/icons

But be sure you click on FREE to get the icons that can be (not must be!) available on the J51 templates.:



And last but not least: be sure you have the fontawesome icons allowed in your template under General > Typography Styling > right side on the bottom.

Hope this helps
Regards
Oliver