how to sum all td values ?

That's because you are assigning individual value to the field, not sum. Haven't tested it, but something like this should work:

Code:

<script>
function updateTotal()
{
    var total = 0;
    $('#dynamic-table tr').each(function() {
        if (!this.rowIndex) return; // skip first row
        var customerId = this.cells[4].innerHTML;
        total += customerId;
    }
    $("#grand_total").val(total);
});

Out of curiosity, why are you not doing that on PHP side?