$(document).ready(function() {

    $("textarea").keydown(function(e) {
        if(e.keyCode === 9) { // tab was pressed
            // get caret position/selection
            var start = this.selectionStart;
            var end = this.selectionEnd;
            var value = $(this).val();

            // set textarea value to: text before caret + tab + text after caret
            $(this).val(value.substring(0, start) + "    " + value.substring(end));

            // put caret at right position again (add one for the tab)
            this.selectionStart = this.selectionEnd = start + 4;
            e.preventDefault();     // prevent the focus lose
        }
    });
});