How we can choose only 4 from a group of checkbox when the data is coming from the database

I want to show JavaScript alert when user trying to choose more than four values from the group of checkbox the checkbox names are in array

<div class="checkbox alert alert-primary">

    <p><input type="checkbox" id="calorieCheckbox" name="txt_tradesman[]" value="<?php echo $restrad['tradesman_id'] ?>" onClick="return KeepCount()"><?php echo $restrad['fullname'] ?> <p>    

</div>