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 · Practiced Mastery
Coordinating two-man cover tactics and aggro swaps through scripted firefights.

Optimization · Practiced Mastery
Series finale blending stealth with active battlefields and heavy cinematics.
Army of Two: The 40th Day leans Execution (Practiced Mastery); Metal Gear Solid 4: Guns of the Patriots leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Army of Two: The 40th Day
Cover shooting needs competent aim but generous systems forgive
Metal Gear Solid 4: Guns of the Patriots
flexible gunplay alongside sneaking
Army of Two: The 40th Day leans a little harder on reads, though the gap is small.
Army of Two: The 40th Day
Enemy AI is scripted, co-op partner cooperates rather than opposes
Metal Gear Solid 4: Guns of the Patriots
battlefield factions act on scripts
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Army of Two: The 40th Day
Encounters are scripted with minor spawn unknowns first time
Metal Gear Solid 4: Guns of the Patriots
mid-battle chaos mostly cosmetic
Metal Gear Solid 4: Guns of the Patriots leans a little harder on optimization, though the gap is small.
Army of Two: The 40th Day
Aggro management and weapon upgrades reward planned coordination
Metal Gear Solid 4: Guns of the Patriots
camo, gear, and route knowledge per act
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.