Internal/External name server issue

My company hosts its own website.  We currently run 4 name servers; two external and two internal (primary and backup).

All machines are Windows Server 2008 R2 and are VMs (ESXi 4.1).

Is there a way to cut that down to just two name servers (Primary and backup)?  I need to still be able to let internal users access the website.

Is there a tutorial on setting up a server so it serves both internal and external requests?

I'm not sure what info you need but am happy to provide it.

Thanks for your help.