Copy large files to multiple machines on a LAN

BitTorrent. It's how Twitter deploys some things internally.

http://engineering.twitter.com/2010/07/murder-fast-datacenter-code-deploys.html (web archive link)

How about UFTP, it uses multicast to deliver files over UDP to multiple clients at once. Not for everyone and I'm no expert on it but it sounds like it does what you want.

Have you tried to copy this data with rsync? If you have 1 Gbit LAN or faster, copying over 4*20 GB should not be a problem.

How often will this copy occur? Does it matter if it takes couple of minutes to finish?