This will do it
http://reviews.cnet.com/bridges/netg...-31343070.html

Although it's sold as a print server (and if you want to use it for that, be very careful to check the compatibility list; it's kind of limited in the printers it supports), it also give four ethernet ports.

I'm sure others may have better suggestions, but that one has worked for me flawlessly and it can be found cheap on eBay in the past.

Or currently on Craigslist:

http://norfolk.craigslist.org/ele/1013804228.html
http://sacramento.craigslist.org/sys/1034849274.html