how to add child node to xml?

Hi,

I've got a ribbon.xml, and after "menu", I would like to add button node with its attributes:

 <button id="button4" label="button4" showImage="false" />. this is my xml:

<?xml version="1.0" encoding="UTF-8"?>
<customUI onLoad="Ribbon_Load" >
    <ribbon>
        <tabs>
            <tab idMso="TabAddIns">
                <group id="Messages" label="Messages">
                    <button id="button1" label="button1" showImage="false" />
                    <button id="button2" label="button2" showImage="false" />
                </group>
                <group id="group1" label="Category">
                    <menu id="menu1" label="menu1" showImage="false" />
                </group>
            </tab>
        </tabs>
    </ribbon>
</customUI>

Can anyone help me please

Thanks