There are a few ways you could go when creating a footer that goes with v4.master. One simple method would be to add a div right before the DeveloperDashboard line in v4.master. Add the bolded line below:
<div class="s4-notdlg" style="clear: both; text-align: right; padding: 10px; background-color: #FCFCFC; border: 1px solid #DBDDDE;">
© Copyright 2010 AdventureWorks, Inc.
</div>
<SharePoint:DeveloperDashboard runat="server"/>
<div class="s4-notdlg" style="clear: both; text-align: right; padding: 10px; background-color: #FCFCFC; border: 1px solid #DBDDDE;">
© Copyright 2010 AdventureWorks, Inc.
</div>
<SharePoint:DeveloperDashboard runat="server"/>
