<SharePoint:CssRegistration Name="<% $SPUrl:~sitecollection/Style Library/YourCoreFileNameHere.css %>" After="corev4.css" runat="server"/>
To register more than one CSS external file, check this other post.
Note: I always recommend making copies of Microsoft files and pages and not to work on them directly. For what I described above, I usually make a copy of the v4.master master page and I also make my own copy of the corev4.css file, leaving both the original v4.master and corev4.css files alone.