I think D4 would be Red Faction. After all, it's a pretty short jump from the Descent games to the FPS genre- in Descent you move around in tunnels and shoot any bad guys who show up, in FPSes you move around in tunnels and shoot any bad guys who show up. The very minor functional difference I've seen (and I've played Descent, or at least the first one), have to do with the spaceship piloting thing, which just means you can move around a tiny bit more in tunnels, and have more controls to scrabble at randomly trying to get your dude to move while he is gang-pressed by a half-dozen heavily armed Primitives from Hell.