Quake 4 is a direct sequel to Quake 2, it was made by Raven Software in collaboration with id Software.
It turns out, whilst the main character in Quake 2 won a major success against the Strogg, the war is far from over. Now the Big Gun and the Strogg Emperor have been killed a ground invasion of Stroggos is possible and Earth has attacked trying to do remove the threat of the Strogg once and for all.
It was really interesting playing Quake 4 so close to Quake 2, you can really feel the impact that time and technology has made, the id Tech 4 engine is capable of significantly better interactions and improvements in AI, animation and storage technology means that friendly soldiers not only exist but play an active component in the missions of the game. Admittedly they do restrict the amount of time you spend actively fighting as part of a squad, this is still very much a single man FPS, they justify this by making your character a scout, often being sent on ahead to find and deal with other routes and finding a way to let your squad inside.
What makes Quake 4 so interesting to me is that the enemies you are dealing with haven't significantly changed. Most of the Strogg you fight are recognisably the same or very similar to the Quake 2 enemies, though they have much improved models and more impressive sounds and a few minor new tricks. The crude attempts at horror in Quake 2 where you see the results of Strogg processing humans are painted in entirely new lights. You get a lot more detail in game as to what it entails and how the Strogg control and use their human resources.
Doom 3 and Quake 4 still have the best computer interaction I have seen in first person shooters, the way it smoothly moves between pointing your gun at things and pointing your mouse at interfaces feels a lot more natural than zooming in on the interface and having it fill the entire screen when you press an interaction button. While it results in some laughably simple interfaces, but I feel that the trade off is worthwhile.
Quake 4 also avoids the problems of Doom 3 by spending a lot more time in outdoor environments and areas with nice big windows, it doesn't rely on darkness as much to create tension and lets you see pretty vistas. I really like the approach of using what appears to be concept art as skybox backgrounds, it continues the tradition seen in Quake 2 and it gives you a better idea of Strogg cities and other background elements.
The guns in Quake 4 almost all feel good and filling an identifiable role, you get weapon upgrades at various points through the game, a techie will ask to see your gun, play around with it and suddenly it has a new ability. Unfortunately the Railgun has been significantly nerfed, it has a terrible fire rate and its upgrade (shots go through people) is of such minor utility that it isn't really worth being handed out.
I had a great time with Quake 4, it really helped having only recently played through Quake 2 and having it relatively fresh in my mind to contrast and compare.