How can I allow my servlet to accept 4 parameters via HTTP POST through a URL?
Just invoke a HTTP POST request with those 4 parameters.
Either by a simple HTML form.
<form action="servletURL" method="post">
    <input type="hidden" name="param1" value="value1">
    <input type="hidden" name="param2" value="value2">
    <input type="hidden" name="param3" value="value3">
    <input type="hidden" name="param4" value="value4">
    <input type="submit">
</form>
Or by Ajax (with little help of jQuery).
<script>
    var params = {
        param1: 'value1',
        param2: 'value2',
        param3: 'value3',
        param4: 'value4'
    };
    $.post('servletURL', params, function(response) {
        alert(response);
    });
</script>
Then they'll be available the usual request.getParameter(name) way in servlet's doPost() method.