<textarea type="text" id="myInput"></textarea>
<script type="text/javascript">
    var myInput = document.getElementById("myInput");
    str = "    ";
    if(myInput.addEventListener ) {
        myInput.addEventListener('keydown',this.keyHandler,false);
    } else if(myInput.attachEvent ) {
        myInput.attachEvent('onkeydown',this.keyHandler); /* damn IE hack */
    }

    function keyHandler(e) {
        var TABKEY = 9;
        if(e.keyCode == TABKEY) {
        	insertText(myInput,str);
        	if(e.preventDefault) {
                e.preventDefault();
            }
        }
    }
    function insertText(obj,str) {
        if (document.selection) {
            var sel = document.selection.createRange();
            sel.text = str;
        } else if (typeof obj.selectionStart === 'number' && typeof obj.selectionEnd === 'number') {
            var startPos = obj.selectionStart,
                endPos = obj.selectionEnd,
                cursorPos = startPos,
                tmpStr = obj.value;
            obj.value = tmpStr.substring(0, startPos) + str + tmpStr.substring(endPos, tmpStr.length);
            cursorPos += str.length;
            obj.selectionStart = obj.selectionEnd = cursorPos;
        } else {
            obj.value += str;
        }
    }
</script>