If you want to mass reject them all you can go to the Users Awaiting Approval page, open your browser console (F12 in Windows) and then paste this:
Code:
$("input[value=reject]").click();
$(":checkbox").prop("checked", false);
$("form[action='admin.php?users/moderated/update']").submit();
Then hit the Enter key. Once you do that the form will be populated to reject all the users in the form and the form will be submitted. When the page reload simply re-open your console and press the arrow key up in yor keyboard and hit the Enter key again.
Hope it helps.
P.S. Bath Update Users "works" but I still see those 4500 users awaiting approval in the header and in the Users Awaiting Approval page. Idk if is a bug or the system works like that.