This is a different choice aside from the one provided by @aviboy2006.
To utilize the $ sign, you can include the jQuery path.
<link rel='stylesheet' id='dashicons-css' href='http://bookshark.phlexon.com/wp-includes/css/dashicons.min.css?ver=5.4.1' type='text/css' media='all' />
<!-- ****************** Add in jQuery path ************************************************-->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
<!-- ***************************************************************************************-->
<script type='text/javascript' src='http://bookshark.phlexon.com/wp-includes/js/jquery/jquery.js?ver=1.12.4-wp'></script>
<script type='text/javascript' src='http://bookshark.phlexon.com/wp-includes/js/jquery/jquery-migrate.min.js?ver=1.4.1'></script>
<script type='text/javascript' src='//cdn.jsdelivr.net/npm/es6-promise@4/dist/es6-promise.auto.min.js?ver=5.4.1'></script>
After that, your code will function as shown below: (enclose within $(document).ready(function(){)
$(document).ready(function(){
$("#img1").on('click', function() {
$("#div1").fadeIn();
$("#div2,#div3,#div4").fadeOut();
});
$("#img2").on('click', function() {
$("#div2").fadeIn();
$("#div1,#div3,#div4").fadeOut();
});
$("#img3").on('click', function() {
$("#div3").fadeIn();
$("#div1,#div2,#div4").fadeOut();
});
$("#img4").on('click', function() {
$("#div4").fadeIn();
$("#div1,#div2,#div3").fadeOut();
});
});