In this tutorial you can see how to use font awesome in Joomla 4 module.
Please check code :
//https://docs.joomla.org/J4.x:Web_Assets
//Font Awesome
$wa = Factory::getApplication()->getDocument()->getWebAssetManager();
$wa->registerStyle('joomla-fontawesome', 'system/joomla-fontawesome.min.css', [], [], []);
$wa->useStyle('joomla-fontawesome');