Picked this one up for cheap when I nabbed Prey. For the most part it was a good game with a nice continuation from Quake 2 and a lot more colorful. Q4, Riddick and Prey all have a similar look to me and while I had fun with all of them I think I enjoyed Riddick more. Q4 also reminded me of Aliens in regards to the hallways and mercs you fight alongside.
The best change in Q4 from Q2 and the other games was having good squad AI in most levels. It added nicely to the atmosphere to have other mercs running with you. Both friendly and enemy AI used their surroundings well and would duck and then popup/out to shoot before taking cover again. The game really needed a lean key though.
Decent mix of indoor and outdoor combat. Most of the indoor was infantry though the enemy base. Lots of dark corridors to navigate while eliminating the enemy. While some outdoors was infantry a lot was in vehicles. To me, these sections were the worst part of the game. Getting in a mech one would think it could take some punishment, but then some super enemy launching tons of missiles would appear. Then I'd die while struggling to take out the missiles and still take out the enemy launching them. I much prefered being on foot as while it was sometimes difficult it normally didn't drastically jump from easy to hard like the vehicle sections did. It was also easier as infantry to use your surroundings for cover than in a vehicle out in the open.
At first the mouse control felt sluggish even with sensitivity all the way up, but I got used to it. In a way it made it feel like you had enertia in moving instead of super quick spinning reflexes like many games have. Though of course, the vehicles acted very sluggish, more so than as infantry. Think at the 1/2 way part I started feeling burnt out on it but trudged on a bit and it repicked the pace and interest. Think it was all the tunnel sections where you'd switch from infantry to vehicle and then rinse/repeat with another tunnel and switch again. The game was a decent length without these sections so guess they just liked trying to change up the combat, but again I didn't enjoy the vehicles so I just wanted these sections over.
Still not sure why all these aliens want to harvest people. Its got to be easier to acquire material from lower gravity wells than earthlike planets. They sure like cyborging us into their ships too. Though Q4 wasn't quite as organically weird as Prey.
I didn't try out any mods and probably won't for this game but there are bound to be a lot based on previous ID games moddability.