要为Flex4的Tree组件添加线条，可以使用自定义的itemRenderer来实现。
首先，创建一个名为CustomTreeItemRenderer的MXML组件，并继承自TreeItemRenderer。
CustomTreeItemRenderer.mxml：
```xml
xmlns:mx="library://ns.adobe.com/flex/mx">
override protected function updateDisplayList(unscaledWidth:Number, unscaledHeight:Number):void {
super.updateDisplayList(unscaledWidth, unscaledHeight);
// 绘制线条
graphics.clear();
graphics.lineStyle(1, 0x000000);
if (data && owner && owner.showRoot && data.isRoot) {
// 根节点
graphics.moveTo(unscaledWidth / 2, 0);
graphics.lineTo(unscaledWidth / 2, unscaledHeight);
} else if ((data && data.hasOwnProperty("children") && data.children && data.children.length > 0) || (data && data is XML && XML(data).children().length() > 0)) {
// 父节点
graphics.moveTo(unscaledWidth / 2, 0);
graphics.lineTo(unscaledWidth / 2, unscaledHeight);
graphics.moveTo(unscaledWidth / 2, unscaledHeight);
graphics.lineTo(unscaledWidth, unscaledHeight);
} else {
// 叶子节点
graphics.moveTo(unscaledWidth / 2, 0);
graphics.lineTo(unscaledWidth / 2, unscaledHeight / 2);
}
}
]]>
```
然后，在Tree组件中设置itemRenderer为CustomTreeItemRenderer。
```xml
```
这样，Tree组件的每个节点就会显示相应的线条。
