The problem is, as you know, that the serial port is virtually obsolete in the modern PC world. I doubt if you are going to find a low power motherboard that supports 4 serial ports.
Googling "serial port boards" gave several hits. This looks promising:http://www.quatech.com/catalog/pci_express_lowprofile.php
and prices don't look too outrageous for what is a niche product.
Combine with an Atom CPU and low powered motherboard.