head to head
Neither is simply harder - they are hard in different places. Far Cry 4 demands more optimization; Grand Theft Auto V demands more reads.

Execution · Practiced Mastery
A Himalayan open-world shooter refining the outpost-clearing formula.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Far Cry 4 leans Execution (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Far Cry 4 leans a little harder on execution, though the gap is small.
Far Cry 4
Gunplay plus gyrocopter and elephant chaos
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Far Cry 4
Scripted outpost AI
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Far Cry 4
Wildlife and patrol variation
Grand Theft Auto V
Some mission and lobby randomness.
Far Cry 4 leans clearly harder on optimization.
Far Cry 4
Outpost approaches and skill trees are plannable
Grand Theft Auto V
Heists, money loops, builds 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.