In order to add SyntaxHighlighter to Joomla 4 in System -> Site Templates -> "Cassiopeia Details and Files" -> index.php somewhere on line 133 in head tag I have added two lines of code:
<script type="text/javascript" src="syntaxhighlighter/syntaxhighlighter.js"></script>
<link rel="stylesheet" href="syntaxhighlighter/theme.css">	  
My head block now looks like:
<head>
	<jdoc:include type="metas" />
	<jdoc:include type="styles" />
	<jdoc:include type="scripts" />
	<script type="text/javascript" src="syntaxhighlighter/syntaxhighlighter.js"></script>
	<link rel="stylesheet" href="syntaxhighlighter/theme.css">	  
</head>