The GNU/Linux installer for the Quake 4 demo is on the ftp as well and on the BitTorrent tracker.
Note regarding multiplayer that windows clients are able to connect and play onthe Linux dedicated server, but Linux clients can only play on Linux servers. Read through the FAQ for details.
The demo includes the first two levels of the singleplayer campaign, and two multiplayer maps (large and small versions of “The Fragging Yard”).
Quake 4 Demo Download: [ 3D Downloads | ftp.id | BitTorrent | 3D Gamers | FileShack ]