JSFiddle - React, Tailwind, and code Playground

HTML

<input name="Rate1" type="text" id="" value="1.00" />
<input name="Rate2" type="text" id="" value="1.00" /><br />
<input name="Rate3" type="text" id="" value="1.00" /><br />
<input name="Rate4" type="text" id="" value="1.00" /><br />
<input name="Rate7" type="text" />

JavaScript

$(document).ready(function() {
    $('input').("[name=\"Rate1\"]").keyup(function() {
        var a = $('input[name="Rate2"]').val();
        var b = $('input[name="Rate3"]').val();
        var c = $('input[name="Rate4"]').val();
        var d = $('input[name="Rate1"]').val();
        if ($.isNumeric(a) && $.isNumeric(b) && $.isNumeric(c)) {
            $('input[name="Rate7"]').val(a * b * c * d);
        }
    }).eq(1).keyup();
    $('input[name="box4"]').attr("disabled", "disabled");
});