The Art and Design of Quake 4 is a GameSoy Q&A asking Eric Biessman and Kevin Long of Raven Software, and Tim Willits of id Software about the style of the game.

Sluggo: Do you think characters and weapons will be easily recognizable to people who played Quake 2, or is the DOOM 3 engine just so far ahead that it's night and day?
Kevin Long: I think you can. The updated Strogg marines are a little different but you can still tell they're the Strogg marines. The gladiators have somewhat the same feel, but are also getting updated, a little bit more streamlined, to create a model in hi-poly and show a lot of bolts, compared to the Quake 2 stuff which was a little lower poly. I think you can tell that it's a Strogg facility and that some of the creatures are returning creatures from Quake 2.