Problem on init data to textarea

hi jogeter...

i found problem on init data to textarea, but i can't explain much

i can give the sample of my project 

this picture of xpdl

this is form layout:
input comment : textarea_4
history comment : textarea_1

<style type ="text/css">
	#_ex_textarea_1 {height:300px;}
	#_ex_textarea_4 {height:50px;}
</style>

<script type="text/javascript">

	var Data_history

	function formbuilderOnReady() {
		Data_history = document.getElementById("_ex_textarea_1").value;

	}


	function formbuilderCustomValidation() {

		var var1 = document.getElementById("_ex_textarea_4").value;
		var display_history = "\n"+"------------------" + " [#date.dd'-'MM'-'yyyy hh:mm#] #assignment.assigneeId# " +"\n"+"komentar : " + var1+"\n"+ Data_history +"\n";
		document.getElementById("_ex_textarea_1").value = display_history;

		$('#_ex_textarea_4').attr('value','');

	}


</script>

the problem found when we update comment repeatly

the history comment has missconfiguration on init data from database

for example :

on activity 1
type on input comment :
test 1

on activity 2

type on input comment :
test 2

on activity 3

type on input comment :
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

then : see result on history comment on activity 4

look like misconfig or uncorrectly on init data from databse to textarea_1

any idea to solve this problem