In 1997 Quake II was released ending the Strogg's collective leader the Makron being killed. Quake III moves away from the Strogg, but the universe's worst cybernetic foes return in Quake IV, ready to be destroyed yet again by Earth's forces. Quake II left the Strogg home planet without its planetary defenses, leaving it open for your attack as part of Earth's new armada. You help deliver the final assault to the Strogg as Matthew Kane, member of an elite squad fighting along squadmates with vehicles and advanced weaponary. Quake IV is based on the Doom III engine, although it is apparent that Quake IV will be a superior modding platform thanks to iD and Raven's efforts in optimizing the netcode, adding vehicles, and general fixes made to the engine since Doom III's release.
Quake 4 is a good follow up to Quake 2, where the storyline is somewhat following where Quake 2 left off. For a single player game Quake 4 is really good and entertaining, and will provide several hours of fun.
But if you look at the multiplayer perspective of the game, it has a lot to learn from former games and other games within the genre. The game overall has a lot to offer, tons of fun but also isn't reaching it's full potential. This is where mods like Q4Run, Q4Max, GameType-Revolution and XBM come handy to enchant the overall game experience. Still I would say it's worth the money and would recommend it to Quake players as it has something unique like every Quake game before it has had.