If you've occasionally thought to yourself "I'd like to play the worst Quake game, but in the engine of the second-worst Quake game", then do I have the mod for you. "Quake 4 in Quake 2" takes Raven Software's somewhat maligned attempt to bring Quake into the noughties, and remakes all of its levels using Quake 2 assets.
It's a full-on demake of Quake 4's campaign into the Quake 2 engine, taking you from the Call of Duty-like opening missions where you play as a space marine, to the more classically inspired FPS levels of the game's post-Stroggification second half. The mod's creator, Benjamin Pudlio describes his recreations of the Quake 4 maps in Quake 2 as "similar". You can get an idea of how similar they are in the comparison video below.
Alongside a full conversion of Quake 4's campaign into the Quake 2 engine, Quake 4 in Quake 2 also includes a new graphical interface, weapon and monster sounds from Raven's game, and "remastered music from the official expansions" (presumably referring to Quake 2, since Raven's game didn't have any expansions). It also appears to be a slightly modified version of Quake 2. I don't recall Quake 2 being quite so liberal with the blood decals.
The big question is, of course "Why?". Although I personally like Quake 4, I don't think anybody was crying out for a new version of it, let alone a demake. Pudlio doesn't offer any explanation behind the project, but my guess is it's to create a version of Quake 4 that's more in-line with id's original vision of its stroggified universe. Some of Raven's creative choices were controversial at the time, not least making the game a more Call of Duty-like squad-based shooter. So perhaps there is an audience for a more "authentic" version of Raven's sequel to Quake 2.
In any case, you can download Quake 4 in Quake 2 here. Note that you'll need to download Kmquake 2, or update vanilla Quake 2 with the unofficial patch 3.24.