head to head
Neither is simply harder - they are hard in different places. Persona 4 demands more variance; Portal demands more execution.

Optimization · Pure Optimization
Small-town murder-mystery JRPG balancing friendships by day and TV-world dungeons by night.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Persona 4 leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Persona 4
Menu combat; zero dexterity
Portal
Portal placement timing modest.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Persona 4
Shadow bosses use scripted patterns
Portal
No adversary modeling player.
Persona 4 leans clearly harder on variance.
Persona 4
Randomized dungeon floors and fusion rolls
Portal
Deterministic chambers; no randomness.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Persona 4
Social link scheduling and weakness-exploit planning
Portal
Pure puzzle-solving optimization.
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.