Based on your diagram I'm not sure why you need 4 IP's. x.x.x.81 could be your single point of presence. Just setup the firewall to forward port 80 to your web server, port 25 to your e-mail server, port 53 to your public DNS server. This won't work if you have multiple servers running the same service, websites for example that would need port 80. So, let me assume you have more going on than you explained...
First, what version of Endian are you using?
In version 2.4, which I'm most familiar with, you can walk through the "network configuration" wizard (if I can call it that) and specify multiple IPs for the red interface. They need to be entered in a format of x.x.x.1/24, where 24 represents your subnet mask, in the additional IP's box.
^^Agree, If you have 4 red nic cards you can also assign them under network settings, but that is normally used for redundancy or static routing, although they may do what you are trying to do to, which maybe to access different public ip's for each of your servers, but you will still have to use port forwarding internally as stated by hickmanr... I don't see why you would not just use 1 public IP, unless its a bandwidth issue, or maybe you are trying to set up so you can remote to your servers externally, in which case you can configure the remote service port to different ports other then default and just forward those specific ports to each server so 1 ip works...
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