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"/>
This would add a footer below the left nav and body that matches the colors in the default left nav. You could of course expand on this by moving the stlye to an external style sheet or changing the look and feel.