head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is reads, and it is a single point.

Execution · Competitive PvP
Fast aim duels plus predicting human movement on memorised maps.

Execution · Pure Mechanics
Fast arena-style shooter mixing a scripted Strogg campaign with twitch multiplayer.
Call of Duty: Modern Warfare Remastered leans Execution (Competitive PvP); Quake 4 leans Execution (Pure Mechanics).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare Remastered
Snap aim and reaction time dominate every gunfight
Quake 4
Aimbot plus strafe-jump inputs dominate both campaign and multiplayer
Call of Duty: Modern Warfare Remastered leans a little harder on reads, though the gap is small.
Call of Duty: Modern Warfare Remastered
Online opponents rotate and adapt; pre-aiming them is prediction
Quake 4
Multiplayer duels reward predicting human positioning and timing
Call of Duty: Modern Warfare Remastered leans a little harder on variance, though the gap is small.
Call of Duty: Modern Warfare Remastered
Spawns and random lobbies inject per-match noise
Quake 4
Spawn and item timing add mild uncertainty
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare Remastered
Map control, loadouts and timing form a learnable meta
Quake 4
Item control routes and weapon picks are learnable
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.