
  What you need is a hardware router such as EtherRoute/TCP made by
Compatable Systems ($1400).  This will allow you to connect your Localtalk
network (4 Macs) to your Ethernet network (NeXT and Ethernet Mac).  It will
route TCP/IP protocol between the two networks.  Software routers are also
available for less money, but I'm not sure if they work with TCP/IP.