head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Zombie Army 4: Dead War demands more execution.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.

Execution · Practiced Mastery
A co-op horde shooter about sniping waves of occult Nazi undead.
Grand Theft Auto V leans Execution (Practiced Mastery); Zombie Army 4: Dead War leans Execution (Practiced Mastery).
Zombie Army 4: Dead War leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Zombie Army 4: Dead War
Long-range headshots and horde crowd control
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Zombie Army 4: Dead War
Undead waves never adapt
Zombie Army 4: Dead War leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Zombie Army 4: Dead War
Spawn variation and elite placement shuffle runs
Zombie Army 4: Dead War leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Zombie Army 4: Dead War
Loadout perks, trap placement, score-chain knowledge
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.