This game feels a bit different from Quake 1 and 2, the first half feels more like a Call of Duty game than a Quake game, but the second half more than makes up for it with big guns and bigger monsters. I believe Raven's intention was to capture the Halo-style console FPS players by starting the game off slow with heavy reliance on the assault rifle and shotgun while fighting mostly humanoid enemies in some not-so-alien locations, then slowly ramp up the tension until they're fully into the Quake mojo. At least, that's what happened to me when I played it as a teenager
The atmosphere, especially in the second half of the game, is wonderful. It's dingey, rotten, and full of disturbingly twitching human body parts hooked up to arrays of machines. In some of the Strogg areas it can be seen that headless and limbless human torsos are being kept alive to act as batteries for Strogg machinery. One of the areas near the middle of the game is frozen and it's got a beautiful blue atmosphere with bright orange highlights. It's hands down one of my favorite levels in any videogame ever purely for the atmosphere
The music is okay, not amazing but it keeps the mood up while shooting Strogg.
The weapons are a mixed bag. Some personal favorites are the shotgun, assault rifle, hyper blaster, rocket launcher, and grenade launcher. The Nailgun is like a slow but extremely powerful chain gun that eventually gets upgraded with a scope and homing nails
The enemies are creepy humanoid monstrosities that seem like an excellent upscaling of many of the Quake 2 Strogg designs, though with a more sleep design sensibility as opposed to the extremely chunky and industrial designs of Quake 2
The vehicle sections range from boring to awful, with a near constant ear grating machine gun sound or almost as constant ear piercing missile warning alarms. Thankfully they're short and easy
I love Quake 4, I've come back to it repeatedly over the years and I will continue to do so
That one part though...