QuakeHub.net

1. Buy & install Quake 4 on Steam.
2. Make the in-game server browser work. Quake 4's official master server no longer serves a server list, which is why Multiplayer → Internet is empty. Put seta net_master0 "master.quakehub.net" in an autoexec.cfg inside your q4base folder and the list fills up. We run that master and the code is open source, so anyone can host their own.
3. Most servers run Q4Max. It is the competitive mod the scene standardised on, and the large majority of live Quake 4 servers use it - stock Quake 4 can only join the rest. Quake 4 does try to pull mod files down when you connect, but it is rate-limited and routinely fails on a full map pack, so it is far less painful to have it before you join. Grab Q4Max 0.82.
4. The easy way to get the files: PlayQ4-net's launcher bundles Q4Max 0.82 and every competitive map in one download. Use it to get set up, then find servers here or in the in-game browser - the launcher's own list is a short hand-maintained one and most of it no longer answers.
5. openQ4 is an open-source engine port for modern systems, Linux and Steam Deck. It plays online fine, but its server browser can't be pointed at a master, so find servers here and connect by IP.

Joining a server Set net_master0 as above and join from the in-game browser; from PlayQ4's browser in one click; or from the console, connect ip:port.