Try something like this:

VB Code:
  1. e.Item.Cells(4).Style("cursor") = "hand"
  2. e.Item.Cells(4).Attributes.Add("onclick", "javascript:assignValue('textbox1', '" & e.Item.Cells(4).Text & "')")

Code:
<script type="text/javascript">

	function assignValue(ID, assignedvalue)
	{
		document.getElementByID(ID).value = assignedvalue;
        }

</script>
Haven't tested it or anything, so not sure if it will work, but its a start