head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is execution, and it is a single point.

Execution · Pure Mechanics
Flawless execution of memorised patterns under one-hit-death pressure.

Execution · Pure Mechanics
Retro Studios' notoriously tough revival of Donkey Kong Country with relentless set-piece levels.
Contra 4 leans Execution (Pure Mechanics); Donkey Kong Country Returns leans Execution (Pure Mechanics).
Contra 4 leans a little harder on execution, though the gap is small.
Contra 4
One-hit deaths under dense bullet patterns demand near-perfect inputs
Donkey Kong Country Returns
Demanding rocket-barrel and mine-cart sections, strict timing
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Contra 4
Every enemy and boss is a fixed script
Donkey Kong Country Returns
Scripted hazards, no adaptation
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Contra 4
Spawns and patterns are identical every run
Donkey Kong Country Returns
Hidden puzzle pieces, fixed once found
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Contra 4
Memorising layouts and weapon routing enables the execution
Donkey Kong Country Returns
KONG letters and time attack routes are 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.