head to head
Neither is simply harder - they are hard in different places. Gran Turismo 4 demands more execution; Grand Theft Auto V demands more reads.

Execution · Practiced Mastery
An encyclopedic driving simulator spanning 700 cars and decades of automotive history.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Gran Turismo 4 leans Execution (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Gran Turismo 4 leans clearly harder on execution.
Gran Turismo 4
Hundreds of cars all demand precise inputs
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Gran Turismo 4
Fixed-line AI offers no adaptive opposition
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Gran Turismo 4
Deterministic outside slight tyre wear modelling
Grand Theft Auto V
Some mission and lobby randomness.
Gran Turismo 4 leans clearly harder on optimization.
Gran Turismo 4
Event-by-event car and tune selection is deeply solvable
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.